wireless TTL and metering mode

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mbpautz762
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p.1 #1 · wireless TTL and metering mode

I just have a quick one for everyone here, something I've been confused about lately.
I have a job photographing tanker truck parts in a light tent for a company's website. I've been using manual triggers for off camera flash with perfectly fine results (after some trial and a lot of error :/). However, with over 10,000 parts to photograph and catalog, I recently got a nice fancy phottix odin system for wireless TTL in the hopes that I could speed up my workflow. basically instead of taking a test shot or two before getting the right exposure, I can now pretty much nail it each time, thus saving shutter clicks and time.

Anyway, some of the more reflective silver/metal parts obviously overexpose, which I control with negative exposure compensation. But I noticed it happens no matter what metering mode I use. I thought I would be able to spot meter on the part itself and the TTL would adjust the power level accordingly to expose that spot perfectly, but that is not the case. Does metering mode have any effect whatsoever with wireless TTL flash?

BTW I typically shoot manual mode, 1/125, f/16 - f/22, ISO 400 with two flashes: one above the light tent shooting down, and one in a small softbox to fill in the front of the part. A lot of these are macro shots.

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Nov 29, 2012 at 09:10 PM

Steve Wylie
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p.1 #2 · wireless TTL and metering mode

I may be wrong, but I think the answer to your question is "no". TTL means that the flash is metering the scene and working to give you what it calculates to be a decent exposure. In the example above, it has done its job pretty well. The highlight on the screw threads (I assume that's your concern) still shows detail; I don't think anything is clipped.

What I don't quite understand is why you would prefer a TTL approach when your catalog shots will be in the same environment under the same lighting conditions. I would think that your exposures would be consistent throughout, with minor adjustments for key light placement if necessary, depending on the part to be photographed.

Nov 30, 2012 at 03:38 AM

mbpautz762
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p.1 #3 · wireless TTL and metering mode

mainly it was because the parts vary drastically from jet black gaskets to pure white plastic parts and everything in between, including 300lb hydraulic pumps

I thought the TTL would speed things up versus manually playing around with each individual product, but I sort of wanted to just experiment too.

Since I have you here, for a product like what's shown, what do you think the optimal light placement would be? I'm new to this kind of stuff and am curious what the best way to do it is. I am using two Speedlights and might add a third. Right now, I have one shooting directly down into a large light tent from above, and a second in a softbox to fill in the front. I would like to eliminate the shadows at the bottom, but the company doesn't have the light table type stuff I would need, and some of these parts are very large and heavy. I am currently just using the diffusion material that came with the tent.

Nov 30, 2012 at 04:07 AM

BrianO
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p.1 #4 · wireless TTL and metering mode

For product shots, my preference would be manual exposure and manual flash power, with both parameters established by a flash meter. Once your lights are in place, I would assume that you don't move them much -- if at all -- during a session, so metering once and then leaving everything alone would make for a fast and efficient work flow.

The problem with TTL metering is that the camera doesn't know if a part if black, white, gray, bronze, steel, etc., so it will try to make everything look middle gray...probably not what you want. By using a flash meter, you wouldn't need to make multiple test shots; because you'd be measuring the light falling on the subject rather than the light reflecting from it. You meter once and you're done.

If you do still want to use TTL metering, the way to prevent blown highlights is to spot-meter the highlight: take a shot with the lights positioned, check the LCD to see what's blown, put the metering circle over that spot, trigger flash exposure lock, recompose for the desired framing, then take the shot. It's faster than it sounds, once you've done it a few times, but still much less efficient than taking an incident reading.

The problem will be that while your highlight area will now be exposed for medium tone, your shadows and mid-tones will likely be underexposed and so you'll still have to fiddle with flash exposure compensation.

My final suggestion, if you haven't already done so, is to get the book Light -- Science and Magic. It has many sections devoted to how to light objects to avoid unwanted reflections and specular highlights...and also why some can be a good thing for revealing shape and texture.

Edited on Jan 04, 2013 at 08:30 PM · View previous versions

Nov 30, 2012 at 04:22 AM

BrianO
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p.1 #5 · wireless TTL and metering mode

mbpautz762 wrote: ...Since I have you here, for a product like what's shown, what do you think the optimal light placement would be? ...I would like to eliminate the shadows at the bottom, but the company doesn't have the light table type stuff I would need, and some of these parts are very large and heavy.

Two large soft boxes, one to each side of the camera and as close to the lens axis as possible would be one way. A ring light would be another. Either would give a very flat lighting that would essentially eliminate the shadows under the part. The problem is that such flat lighting doesn't show shape very well. Uplighting the part on a translucent stage is better, because that's seperate from the subject lighting, but I understand that such a stage may not be feasible for your heavy products.

An alternative is to take two exposure, one metered for the subject and the other over-exposed to blow the background, and then combine them in post processing. With 10,000 parts that could be a bit time consumming, but it is an option.

Nov 30, 2012 at 04:29 AM

Steve Wylie
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p.1 #6 · wireless TTL and metering mode

I really think that a consistent manual exposure with camera and flash is the way to go. Anything different will give you a range of background "whiteness" that will be really distracting in a catalog. As for the shadow, I wouldn't try to get rid of it. It provides a base for the part, as opposed to rendering it floating in space. I think your example is very good as is. The only thing I would consider changing from part to part is the placement of the frontal fill light, depending on the shape of the part, if even that.

Nov 30, 2012 at 04:33 AM

mbpautz762
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p.1 #7 · wireless TTL and metering mode

ah! That makes perfect sense. First, for the TTL, I was using exposure lock, not flash exposure lock, a silly oversight now that I think about it.

I do not own a flash meter, but may invest in one now. Say if I go back to manual flash and use a light meter. I would still need to take a new reading when moving from a black part to say, a steel one, and adjust the power levels accordingly right? Because even though between the two parts, the light falling on them is the same, but the reflected light that creates the exposure in camera would be different?

Nov 30, 2012 at 04:36 AM

BrianO
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p.1 #8 · wireless TTL and metering mode

mbpautz762 wrote: ...Say if I go back to manual flash and use a light meter. I would still need to take a new reading when moving from a black part to say, a steel one, and adjust the power levels accordingly right? Because even though between the two parts, the light falling on them is the same, but the reflected light that creates the exposure in camera would be different?

If you want a black part and a steel part to both look gray, yes, you'd change the exposure for each one. But if you want black to look black and gray to look gray, then you meter the light -- not the part -- and shoot accordingly. You may need to boost the black part's exposure a bit, to get recordable texture, but not as much a a reflected-light reading would have you believe, since reflected-light meters assume that whatever they're pointing at is 12-18% gray.

Nov 30, 2012 at 04:50 AM

mbpautz762
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p.1 #9 · wireless TTL and metering mode

thank you all for your help. This is a newbie question, but can you clarify about the 18% gray? so the exposure system always tries to make things gray. Relating to the histogram (since no light meter yet), does that mean that a histogram (at least for a simple object) that is clustered entirely in the middle would end up appearing gray? I noticed earlier photographing very dark objects, if I try to bump it to where the histogram shows a traditional bell curve or is clustered in the middle, it is wayyy overexposed. What looks right as a finished shot is when the histogram is clustered around the lower third. So the camera is showing underexposed but that's only because it THINKS it needs to be gray instead of black. is that correct?

Nov 30, 2012 at 02:54 PM

Steve Wylie
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p.1 #10 · wireless TTL and metering mode

Basically, yes. Your histogram shows the distribution of pixels in terms of their luminosity, where 0 equals black and 255 equals pure white. The midpoint, roughly 128, equals somewhere between 12% and 18% gray (this would be for a black and white image; a color image can still be considered to have the same luminance value as "18% gray" even though it's not "gray"). A standard bell curve doesn't represent reality; things can be dark or bright, and there's no reason to force them to the middle. What you want to avoid is a significant distribution of pixels blocking up on the left or the right, representing clipping of shadows or highlights (clipping means no apparent detail in those areas). Bottom line: use the histogram to avoid problems like clipping, but don't use it to force your exposure into something arbitrary. Again, look at your original sample above; its histogram would show a preponderance of values to the right, but without clipping, and the part you photograph will be somewhere close to the middle. It will not be a standard bell curve, nor should it be.

Nov 30, 2012 at 03:26 PM

BrianO
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p.1 #11 · wireless TTL and metering mode

mbpautz762 wrote: ...I noticed earlier photographing very dark objects, if I try to bump it to where the histogram shows a traditional bell curve or is clustered in the middle, it is wayyy overexposed. What looks right as a finished shot is when the histogram is clustered around the lower third. So the camera is showing underexposed but that's only because it THINKS it needs to be gray instead of black. is that correct?

That's exactly correct. If you don't have an incident light meter, one way of getting more-accurate starting exposures is to fill the frame with a gray card and meter the reflected light from that. For product photography you can bring in the gray card, use the FEL (on Canon cameras) to fire an ETTL pre-flash and meter it, then remove the gray card and take your product shot. As long as you don't change the light-to-subject distance or the aperture, the exposure will be technically correct. It may not be aesthetically correct, but will be a good starting point.

For manual flash/strobe, you'd take a test shot of the gray card as a stand-in for the product, and adjust the power of the lights to get the histogram in the middle, then use that power level as the starting point for the product shots.

Nov 30, 2012 at 07:53 PM

BigIronCruiser
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p.1 #12 · wireless TTL and metering mode

You mentioned FEL. Does that actually work reliably with the Phottix triggers? It's not currently supported by the PW Flex and Mini products, so just wondering.

Dec 03, 2012 at 03:22 AM

BrianO
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p.1 #13 · wireless TTL and metering mode

BigIronCruiser wrote: You mentioned FEL. Does that actually work reliably with the Phottix triggers? It's not currently supported by the PW Flex and Mini products, so just wondering.

I don't use any of the above, so I can't answer that question; I use a Speedlite in Master mode, or the pop-up on my 7D for controlling my off-camera Speedlites. I was thinking of getting a Phottix Odin, but held off and instead I'll be getting 600EX-RTs.

It seems odd that FEL wouldn't work with the Pocket Wizard ControlTL units, though, since all it does is seperate the metering and sync signals over time. That could be another reason to go with Radio Poppers if it's true.

Dec 03, 2012 at 03:40 AM

BigIronCruiser
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p.1 #14 · wireless TTL and metering mode

FEL seemed to work when using Flex & Mini's with a Nikon D700, but not with my D4. I raised it as an issue with the folks at PW, and they said FEL isn't a supported feature.

Dec 03, 2012 at 06:27 PM

mbpautz762
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p.1 #15 · wireless TTL and metering mode

FEL does seem to work with my Phottix Odin system, but it doesn't seem to be as reliable as when using a mounted TTL Nikon SB-910

Dec 03, 2012 at 08:37 PM

Michael White
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p.1 #16 · wireless TTL and metering mode

If the flash to subject distance isn't changing then use manual mode on the flash and each and every shot should be lite with the same amount of light. So if it was me I would setup my lights in soft boxes and position them at a certian distance that allows the largest part to be lite without the soft boxes showing in the frame and allow me to reposition the part then switch out to the smallest part center it in the area that the larger part now the subject to flash distance is very close to the same better yet once you shoot a Bart make notes of its size, material and your Speedlite settings a nd distance for use the next Imelda a similar size object comes up refer to you notes setup everything the same a shoot most of the time it will be your keeper plus by taking the notes and referring back to them you will learn the subtitles of shooting these items by the time your done you will know this setting by heart and what to do based on the different finishes

Jan 03, 2013 at 09:59 AM

BrianO
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p.1 #17 · wireless TTL and metering mode

Michael White wrote: ...So if it was me I would setup my lights in soft boxes and position them at a certian distance that allows the largest part to be lite without the soft boxes showing in the frame and allow me to reposition the part then switch out to the smallest part center it in the area that the larger part now the subject to flash distance is very close to the same better yet once you shoot a Bart make notes of its size, material and your Speedlite settings a nd distance for use the next Imelda a similar size object comes up refer to you notes setup everything the same a shoot most of the time it will be your keeper plus by taking the notes and referring back to them you will learn the subtitles of shooting these items by the time your done you will know this setting by heart and what to do based on the different finishes

Huh? Can we get some punctuation in there? And who are Bart and Imelda?

Jan 03, 2013 at 07:39 PM

erichard
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p.1 #18 · wireless TTL and metering mode

I agree with manual flash, but another way to do it is to get one of the brighter, more specular pieces and push the exposure higher (to the right on the histogram) until you see clipping. The clipping may or may not matter to you for the really specular highlights, but assuming it did, use that exposure for all the pieces. One fine point to this is you can actually go up a 1/3 or 1/2 stop above the point where there is no blinking light on the LCD (the clipping signal) because the LCD is 8 bit, whereas many (most) professional level cameras are a higher bit depth .. 12bit, 14bit, which means more highlight data is preserved in the file than what you see on the LCD. So even with the very initial signal for clipping, you can leave the exposure there (test it out by viewing it in Photoshop or Lightroom if it makes you nervous). And then I'd leave the setup the same for all the products, as long as they fit the setup, so as to keep the background even from shot to shot (without having to do much in post). If you have a 300lb piece that doesn't fit, then you need to adapt of course.

The above will work, will preserve you highlights, and will max out the data in your files. I would think it'd be the way to go with speedlights, being fast, easy, and relatively fool proof.

Having done the above, with your highlights preserved, in Lightroom I think I'd up the "highlights" significantly, while pulling back the "whites", such that the background becomes more white while at the same time preserving the highlights. Basically pull up the highlight slider (or use curves) until the background is as white as you'd like, and then pull back the "white" slider until the clipping signal goes away (I use the right triangle in the histogram as the signal). Same could be done in Photoshop, but the easy part of Lightroom is that you could quite easily apply this to all the shots at once with a sync, just as you would white balance, etc. I haven't done what you're doing with 10K parts, but I suspect it'd be pretty easy (10K of anything notwithstanding), not involving much post work.

I'd be better in RAW, but here's a manipulation of the above photo with highlights pushed up 100% and the white slider pulled back 43% (second photo):

This image is copyrighted by the owner

I think the whiter background looks less muddy than the grayish original one. To me it looks more professional. It'd be hard to accomplish this simply with lighting. I think you need some post processing, as it allows you to compress the data on the right without blowing the highlights.

With studio strobes, I'd use a flash meter for an incident light reading, as it would be fairly easy to do.

Jan 03, 2013 at 08:08 PM

Steve Wylie
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p.1 #19 · wireless TTL and metering mode

Good advice here. It's going to be very important, and very beneficial, to keep your white background consistent. Once you dial that in, whether by locking in your exposure, or by making Lightroom or ACR adjustments (as described above), note how you did it, so you can replicate it time and time again. Your parts will be consistently exposed regardless of what they are. The fact that you're photographing these parts on a white surface can really help you in terms of exposure as well as white balance. Look at the "info" panel in Photoshop to see that your luminance values on the white areas are just below 255 across the entire white surface, and that they match in R, G, and B values (whatever they are), and you'll be set.

Jan 04, 2013 at 01:05 AM

cgardner
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p.1 #20 · wireless TTL and metering mode

Digital sensors and color film both have fixed ranges they can record. To record the fulll range of detail perceived by eye with flash the key:fill ratio must be adjusted until the same detail is seen in the shadows and highlights at the same time in what the camera sensor: fitting scene range to sensor range.

With Canon ETTL it's just a matter of adjusting FEC until solid white highlights like paper are below clipping and reading 250.250.250 with the eye-dropper then adjusting the A:B ratio, with the master on the camera in A group as fill and slave off axis in group B as key until you find the ratio which give you the desired shadow detail.

The first test I did after getting my pair of 580ex flashes in 2004 was to set up a target with white, black and gray 3D objects and try every ratio from 1:8 to 8:1, keeping the exposure adjusted with FEC for the highlights to see what each looked like as captured in the playback and on the computer and which of the ratios recorded the full range as seen by eye. For my camera A:B = 1:2 (slave key 2x stronger that master fill) recorded the full range so that became my starting "baseline" when shooting: set ratio for 1:2, adjust FEC for highlight detail, evaluate then adjust as needed to account for variables such as one room bouncing more light around than another. The ratio needed to record a full range that will vary depending on the range of the sensor and from location-to-location due to factor like "spill fill" bouncing off the walls and ceiling if the footprint of the flashes hit them. Note that when changing A:B ratios in ETTL mode you may need to also tweek the FEC to keep the solid whites at 250 because the metering, while good when changing ratios, isn't flawless.

With manual flash the goal is the same, matching the full black-to-white detail in the objects to the sensor range. Set the aperture for the f/stop needed for the desired DOF then with the key light off raise the manual power of the fill flash until you see the desired shadow detail in the playback of the camera and the histogram stops running off the left side. That takes care of the shadow detail. Now gradually raise the off axis key flash until the white areas it hits are just below clipping. That takes care of the highlight end.

The middle tones? The camera is engineered to create a linear "seen by eye" progression in the tones so when you get the ends of the tonal scale accurate everything in the middle will be also look similar to what is seen by eye. In other words if you were to shoot a bride and groom holding a grey card and filled for detail in the shadows in the groom's suit and adjusted the key light for detail in the bride's white dress the faces and the gray card would look similar to how you'd normally see them. Not perfectly perhaps due to the limitations of the technology but very close.

Of the two modes Manual is more consistent from shot-to-shot and better suited for static lighting set-ups because there is no metering by the camera controlling the flash power as with E-TTL mode. E-TTL is better suited for tasks like running around a room taking "snap" shots where things like consistent tone in the background are not critical.

Once you have recorded detail in the end points accurate at capture in terms of detail, fitting scene range to sensor range, there are many ways to tweek the middle in ACR and Photoshop. With curves or the middle slider in levels, adjustment layers, dodging/burning and many other methods to globally and selectively make mid-tones between white and black a bit lighter or darker.

Jan 04, 2013 at 11:27 PM

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