I can understand the difference between f and t stops, but is there any reason the number of pieces of glass in a lens would effect depth of field? I'm convinced my 35mm at f/2.8 has noticably less dof than my 24-70 at the same aperture.
I think Focal Length has more to do with it than elements, that and lower aperture. The 24-70 will have less DOF at 70mm/f2.8 than the [email protected]
Not sure which will have more at the same FL though. I'm interested to hear the facts on this.
Shouldn't take too much to set up a test shot with a ruler and a tripod.
I would venture to guess that the following is going on:
1. the two lenses are not reporting aperture correctly... and by that I mean that 2.8 for one isn't identical to 2.8 for th ohter. For instance I remember reading that Sigma's 85 1.4 isn't really 1.4
2. the FOV markings on the zoom may not be entirely accurate either ... so maybe when you think you're @35mm on the 24x70 you're really more like @32mm
3. the two lenses may have a different AF performance characteristic. Since DOF is a function of hte focusing distance I suppose that if you were backfocusing a tad on the 24-70 the DOF would look wider.
the three things combined could account for your perception. That or you just need to drink less while shooting.
My guess is that it is due to the additional elements as they are needed to hit the sub 2.8 range. With those elements in place, the image is bound to get more blurry outside the focused range due to the bending light. This comes across as additional DOF.
amonline wrote:
My guess is that it is due to the additional elements as they are needed to hit the sub 2.8 range. With those elements in place, the image is bound to get more blurry outside the focused range due to the bending light. This comes across as additional DOF.
Jim, there's a HUGE framing difference between the two. Either you were NOT on a tripod (or the tripod moved) or indeed one of the two lenses lies when it says it's 35mm
lisy78 wrote:
Jim, there's a HUGE framing difference between the two. Either you were NOT on a tripod (or the tripod moved) or indeed one of the two lenses lies when it says it's 35mm
I did not use a tripod but I had the camera set on the edge of my pool table. I set the mm on the 28-75 according to the markings on the lens so it could be off a bit.
amonline wrote:
HAHA I thought the exact same thing when I saw that.
Yeah.
The 28-70 is a good lens but not great.
At 2.8 the 35L is so sharp it can cut marble that has not been quarried yet.
DOF can change, its lens dependant, not F-stop dependant. It shouldnt be too much, but in real life some F2 lens have DOF like F1.4 and some like F3.5. Might be tied to number of elements, but it might be also tied to fact that official focal length isnt always real focal length. (eg. lens can have 186mm and advertised as 200mm, or should be 25mm but in real life its more like 27mm).
I think that especially at wide-angle settings, zoom can be bit inaccurate as far as focal length settings go.
Transmission part - its pretty consistent, so it probably really is f2.8 with it appropriate light gathering capability, but it doesnt say how much DOF it has. Metrics for 35mm f2 are unfortunately unavailable, so no clue if it transmits more or less at f2.8.
I think it has far more to do with the design and grouping of the elements than the number of them. Also keep in mind the actual focal length of certain lenses may be slightly less or slightly more than what is indicated. This is true for both primes and zooms.
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