[D1N0]

I came across this discussion on BHP Variable Apertures and Depth of Field | explora

Basically they are saying that with a variable aperture zoom lens you always have the same aperture DOF-wise but not light-wise. So the e.g. the DA 50-200 F4 5.6 will have 200mm F4 DOF but 200mm F5.6 light at 200mm because you lose light zooming in. I always assumed that DOF was also F5.6 (since you also change focal length but the aperture's absolute size stays the same) It just sound wrong. Any thoughts on this?

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Last edited by D1N0; 09-16-2014 at 11:58 AM. Reason: Title

[stevebrot]

It does sound wrong in part as you relate it.

• DOF is dependent on absolute aperture (not f-number) and magnification
• Light transmittance is a factor of relative aperture (f-number) and in-lens light loss
• Relative aperture (f-number) is simply the focal length ÷ absolute aperture

The article makes these points explicitly:

• Transmittance is poor for most variable aperture zooms* meaning that f/4 with such a lens will likely provide less light to the focal plane than f/4 on fixed focal length lens
• DOF is the same for any focal length/relative aperture combination as with fixed focal length lenses at the same f/stop.

In other words, there is a potential low-light penalty and also the potential for underexposure if settings are based on a hand-held meter. I don't know why they threw in the DOF part.

Steve

* I don't know why they singled out variable aperture zooms. Almost all well-corrected zoom lenses will have lower transmittance.

---------- Post added 09-16-14 at 12:47 PM ----------

I just re-read your post and it struck me that you are describing the more complicated aspect of relative aperture (f-number) as it relates to zooms. In a simple variable aperture zoom, the absolute aperture remains the same meaning that setting the aperture ring at say f/4 will result in the actual relative aperture decreasing as the focal length increases. The aperture ring is only accurate at one point in the zoom range. So yes, a true f/4 at 100mm would be f/5.6 at 200mm.

Fixed maximum aperture zooms have a mechanism to throttle the absolute aperture down through the range from long to short such that the effective relative aperture remains the same.

Where it gets complicated is when the lens and or body have mechanisms present to insure that relative aperture reflects the set value. The body, after all, has been given the current focal length and maximum aperture by the lens. At least that is potentially the case for all AF Pentax lenses.

DOF for all cases follows the absolute aperture and the magnification.

Here is where I shut my mouth because I have no clue as to what the conventions are.


Steve

---------- Post added 09-16-14 at 01:01 PM ----------

BTW...I just re-read the linked article and the author is poorly informed and a little incorrect. F-number is calculated and is only coincidentally related to transmittance. The reason the f-number changes as the lens zooms is because the focal length changes. The element of uncertainty comes from the definition of "set aperture" as used at the end. If the camera/lens combo supports an accurate relative aperture at all focal lengths as set by the body or the aperture ring then DOF will be consistent with the setting. Transmittance may or may not be depending on lens design.


Steve

[D1N0]

For DOF we need a physical aperture, (the bigger the hole the shallower the DOF). For F number we just need the relationship between Focal length. and physical aperture size 50mm F/2=25mm physical. but in a zoom it's somehow different or not correct. When the 50-200mm has a physical aperture of 12.5mm then at 50mm it would be 50/12.5= F4 and at 200mm 200/12.5= F16 But it is F5.6. The camera does not have a DOF meter just a light meter. So it must get F5.6 light. The DOF is not like a 200mm F16 though. So a zoom lens must act something like a metabones speed booster. Concentrating F5.6 light through the F4.0 through F/16 hole of the aperture. So actually what you need to to in lens design at long focal length's is compress the image to a more compact stream (the DA 560 F/5.6 does not have a 100mm back element. But that still does nots answer the F4 DOF at F5.6 Transmission claim BHP makes. The way I read it they say, The DOF is the same as the widest aperture of the lens, unless you stop down the aperture. So theoretically at 200mm a F4-5.6 should have the same DOF as a 200mm F4 (but a higher shutter speed or a higher ISO). So there is nothing to do but shoot a 200mm F4 against a F4-5.6 zoom at 200mm to prove the point. Am I right?

[stevebrot]

So theoretically at 200mm a F4-5.6 should have the same DOF as a 200mm F4

Yes, that would be the case if the true relative aperture of the zoom for the exposure is f/4. If it is f/5.6, then the DOF will be whatever it is for f/5.6.

There is no magical waving of hands here, just the difficulty of determining what the actual taking aperture is. Efficiency of transmission (f-stop vs. t-stop) is not a factor in DOF calculation, only the aperture size and the magnification. That is where the B&H article gets foggy.

If the camera sets the aperture correctly at exposure time, the DOF will be what ever it is supposed to be for that aperture at that focal length at that distance for that capture format.


Steve

[fgaudet]

Where it gets even more interesting is if you go and do the math in reverse... meaning that if you start with the physically largest diameter of the iris... in the case of the 50-200 this would be at 200mm. For f/5.6 this is roughly 35mm diameter. Now work the logic back linearly down to 50mm... tada!!! f/1.4. Technically this would fit in the lens. If the physical iris opening remains constant. But...

This logic is only true for simple single element lens design. More complicated design which include elements of different shapes, and other various correction will skew the results.

Focal length math is often over simplified using a single element, the same is true with the aperture size where it is always assumed that the aperture blades are sitting in the very front of the lens. Depending where the aperture is located in the lens formula, it may not need to be the theoretical size for the lens actual focal length. Lets say (for example) that the aperture blades are half way in between the first element and the sensor, instead of needing to be 35mm (for 200mm f/5.6) it may only needs to be 18mm. This is partly why diffraction is such an issue, the deeper in the formula the aperture is, the more light will be allowed to bounce back and forth in the lens. This is also why in my 16-50mm f/2.8 lens, the aperture opening is much, much larger than 6mm when at 16mm wide open even if the math says 16/2.8=5.71... Lens optics are more complicated than what most "typical" calculations are meant to represent.

The logic is sound, f/5.6 will still let 2x more light than f/4 and the DOF will be what it should be and the focal length will still be what the number says but the aperture opening size will depend also on the lens design and the position within the groups. So it may not change size but the effect on the amount of light passing through will change as it should.