"Bear in mind that the depth of field of this lens is shallower than other lenses of this FL at the equivalent aperture."
and I read this somewhere:,
"Zuiko lenses generally have much less depth of field than others of equivalent FL and aperture."
I always used to think depth of field was a constant at the same aperture and FL!
However, one of the reasons I gave up my 24-105mm F4L zoom was that I felt it had poor depth of field for landscape use.
I'd like to know more! Which makes have the best and the worst? Are zooms generally worse than primes in this area?
Depth of field is only affected by the focal length of the lens, aperture and focusing distance, not the brand that is printed in the side of the lens. However some manufacturers lenses are not always comparable since forexample "50mm" lens can be anything between 45mm to 55mm which makes a small difference in dof.
edit: also with my experience, sharper lenses tend to create narrower dof because the border of the sharper and blurry part of the image is more visible.
Some lenses have "curvature of field", meaning that the plane of focus is not planar but curved (generally towards the camera at the edges of the frame). Zooms are usually worse, but even some well-regarded fixed focal length lenses have this quite badly. In what follows I will assume the curvature is inwards at the edges, although there are lenses for which it goes the other way.
If you now take a picture of an object in the middle of the frame at shallow depth of field, the lens with higher curvature will *appear* to have shallower depth of field, because the background is in the edges of the frame.
Alternatively, if you take a picture of a smallish room with wide angle lenses such that the side walls are at the edges of the frame, the lens with larger curvature of field will *appear* to have deeper depth of field. I ran this experiment by accident when comparing my Zuiko 21mm f/2 with my Canon 16-35mm f/2.8 Mk II at 21mm using my galley kitchen as test subject. For these two lenses and my kitchen, the effect is worth about a stop.
And then there are those who just see effects where there are none. I can tell you my Zuiko 100mm f/2 shows exactly the same depth of field as my Canon 85mm f/1.8. I would be sceptical about general statements such as the ones you quoted.
You're generalizing too much, Sami. Experience shows that the lens design is a big factor for DOF. It has nothing to do with the actual focal length being different from what is engraved on the barrel. Example: the Zeiss Makro-Planar 50/2 and Planar 50/1.4 have actual an focal length of resp. 51.6 and 51.7mm, according to the data sheets, but at identical apertures (the entire range) the Planar has significantly less DOF (or at least much more background blurring). CarstenW has posted a good comparison that proved this; I can't quite find the link at the moment though (searching...).
Essentially the Zeiss 50/2 MP has roughly a stop more of DOF at comparable apertures than the Zeiss 50/1.4 Planar.
It essentially boils down to the shape of the focal 'plane', which isn't usually a plane at all as you have field curvature. Depending on if the field has a concave or a convex shape will ultimately determine the difference in DOF.
That's one factor. Another is correction for SA. A highly corrected lens will have a very rapid sharpness-to-blur transition while a less corrected one will have a larger transition zone where the SA contributes to extending the blurred area.
So it's not really a very simple parameter and is highly dependent on the lens design.
Jason OConnell wrote:
I may be wrong but i believe sensor size plays a role in apparent depth of field. The larger the sensor the shallower the depth of field.
Yes, that's wrong. What you are thinking of is DOF for equivalent framing. To get the same framing with a smaller sensor you have to move back from your subject, increasing the distance to the focal plane and thus increasing DOF. It has however nothing to do with the sensor but simply that you have to change position of the camera in order to get the same framing.
However, to confuse the issue, the sensor size does matter, but in the other direction - larger sensor = more DOF. This is because DOF is a messy concept that deals not with the projected image but on the final output. The concept 'circle of confusion' is used to describe the minimal part of an image that is acceptably sharp. Different image formats (i.e sensor size) have different circles of confusion if you output the final image to the same size. A larger image format will have a larger circle of confusion and more DOF.
Edit: I'll add an old rant of mine on the subject of why I don't like the DOF concept:
Although I do understand why looking at the CoC from the point of view of a final image is practical, it's also complete bullshit as far as the optical theory goes. No, I'm not saying that the DOF equations that use different CoC:s are wrong, but only that such a use confuses the issue (no pun intended).
The circle of confusion is the diameter of the criterion of maximum permissible unsharpness. I find it thoroughly counter productive to using it when discussing an optical system and it will only be relevant when you have a final image in mind - not what is actually being projected on the sensor, film or whatever.
Let me explain. Suppose we have two sensors A & B that have everything identical except size. A is larger than B. When you use a lens (the image circle covers both sizes) to project an image on the sensors the projected image on B will be identical to the center of the projected image on A if focusing distance, subject and lens aperture are identical. Or put in another way if we take a shot using each sensor, when we inspect pixel by pixel we will find that a cropped version of image A will be identical to image B. Pixel by pixel.
Looking at these images (at 100%) we'll determine some maximum tolerable unsharpness and call it the circle of confusion. It will however be identical in both cases. There will be no difference in the CoC regardless if your sensor is an ultra large format one or a tiny 2/3" as long as the pixel pitch of the sensor is the same. This is of course because the smaller image is simply a crop of the larger one and has not changed in any other way.
Now, when looking at things that way - i.e when not considering the various complex issues that occur when you produce a final image (resize, sharpening, viewing distance, eyesight etc etc) and that are way too complex to predict and compare, using the same CoC regardless of format is the correct way to go.
What's the argument for using the same CoC? The same why we look at 100% crops when we want to see how sharp a lens is. Because resize and sharpen and other operations are bound to affect the final results significantly. We don't say that some lens is sharp on a certain format but not on another. Instead we have MTF charts that are a function of spatial resolution - i.e the MTF are not expressed as a function of line pairs but of line pairs per mm.
So I'm not at all sold on the commonly accepted methodology when it comes to DOF. It seems to me much more reasonable to separate the optical properties of the system to the production (and viewing!) of the final image. A crop sensor does, precisely what the name implies, crop the image. A 135 FF sensor is a crop sensor to a medium format sensor and the latter is the same to a large format sensor. How you crop the image will in no way change the parts you are keeping. Opening an image shot with a 135 FF sensor in Photoshop and cropping the image to the same size as if it had been taken with an ASP-C sensor will give you identical results to an image taken with an actual ASP-C sensor. So why confuse the issue by talking about a later post processed (i.e. resized) image? You certainly wouldn't say that you are reducing the sharpness of an image when you are cropping it, so why would you say that you are reducing the DOF?...Show more →
To answer part of the OP's question, two lenses which carry a lot of depth of field compared to similarly spec-ed lenses are the Zeiss 21 Distagon and 50 MP. The plain Planars have much less.
You certainly wouldn't say that you are reducing the sharpness of an image when you are cropping it
Taking that and the rest of the logic in your post a bit further one should never bother buying a telephoto lens. Just buy an ultra-wide-angle and then crop when you want telephoto results.
In any case, you are wrong in your first post. Every lens design will have a different degree of blur at the same aperture. Some lenses have a lot of depth of field, others much less. The lens design is everything. The DoF calculators are only indications of how much DoF you might get, not accurate calculations. They are based on averages, or the thin lens simplification.
Many lenses appear to be very similar, simply because most lenses are based on the same designs. The double-Gauss design for example. Only those lenses which have significantly different designs will have significantly different DoF, and the two lenses I listed belong to this group.
Personally compare the 50MP@f/2 to the 50 Planar @ f/2 and you will see that I am right. The 50MP has a lot of depth of field for a 50mm lens, even at the same aperture as many other lenses. The 21 Distagon also does. I haven't tested this much, but I am thinking that maybe one or both 25mm lenses do too.
kwalsh wrote:
Taking that and the rest of the logic in your post a bit further one should never bother buying a telephoto lens. Just buy an ultra-wide-angle and then crop when you want telephoto results.
Ken
Ken, that is a fair point, but again, it's looking at it from the output side - something we usually don't do. When we say a lens is sharp or not we generally look at 100% crops. For most optical features the projection on the sensor is the point we look at. After that it isn't about optics any more.
The DOF concept is especially messy as it doesn't take into consideration pixel density and the final evaluation is based on a rather poorly defined appraisal of looking at the output image. Your eyesight will for instance affect DOF as well, to take one example of the complex nature of it.
Sami Ruusunen wrote:
Some people are now confusing dof with a background blur.
In Samuli's comparison shots he also has a set where you can see the difference in DOF very well. Not background blur. There is a very strong relationship between DOF and background blur anyway.
I would say that they are the flip side of the same coin. Sharpness is just a lack of blur, or? Right down to the pixel level. There are of course differences in the quality of the blur from different lenses, but the basic relationship holds.
@Carsten: Do you remember where you posted your DOF comparison shots of the MP50/2 and P50/1.4? I think it was a very good demonstration. Forgot to bookmark it and I can't find it... (but I remember what the shots looked like)
