300mm f4 v 400mm f5.6

You can also use TC's to extend the FL. That also reduces the max aperture (size) and DOF. Plus they introduce their own optical errors (softness, CA, etc).

Is that actually true? The actual lens aperture has not altered all you have done by adding a TC is magnified the image.
 
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You've erred there Steven. If the composition is the same and the aperture is smaller (slower) then the DoF will be greater. DoF is essentially a function of aperture and magnification and focal length is only relevent when distance is the same and composition has been varied as a result. The perspective will of course change if the subject distance changes due to the change in FL and this may make objects outside the DoF region appear more or less blurred.

Bob
I think maybe I wasn't clear... I meant the composition being the same as it would be w/ a crop sensor body (subject distance fixed/not a choice).
If you are using a 400/2.8 and switch to an 800/5.6 (or add a 2x TC) the DOF will be ~ 50% of what it was (2 stops less). And it will be ~ 1 stop less than the 400/2.8 on an aps sensor, which is the composition we are comparing/matching.

FL has 2x the effect on DOF that aperture does... doubling the FL reduces it to 1/4, and doubling (reducing) the aperture doubles it... result is 1/2.
 
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Is that actually true? The actual lens aperture has not altered all you have done by adding a TC is magnified the image.
Yes, if you put a 2x on a 400/2.8 it becomes an 800/5.6 in every aspect. The TC changes the FL of the lens by adding extra elements, but the physical size of the aperture opening does not change; the result is a different FL/Ap ratio (f#).

Or you can look at it as magnifying the lens output simply spreads the light over a larger area (increases the size of the image circle/excess is not recorded). This results in the same amount of light being spread out farther (reduced exposure) and the blur points being larger (less DOF)... but TC's (good ones) are a lot more complex than that.
 
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I think maybe I wasn't clear... I meant the composition being the same as it would be w/ a crop sensor body.

That's what I understood, Steven. If the composition (image) is the same due to sensor cropping and subsequent change in FL then the magnification is unchanged and DOF will be aperture dependant.....smaller (slower in your words) aperture will result in an increased depth of field.

Bob
 
That's what I understood, Steven. If the composition (image) is the same due to sensor cropping and subsequent change in FL then the magnification is unchanged and DOF will be aperture dependant.....smaller (slower in your words) aperture will result in an increased depth of field.

Bob
The magnification is not the same, the image/image circle is larger on the FF sensor.
Try it for yourself... an 800/5.6 on FF has ~ 1 stop less DOF compared to a 400/2.8 on an APS sensor (same subject distance).

Edit: the 800/5.6 would be 2 stops less than the 400/2.8 on the FF sensor (obviously), but the smaller sensor (w/ 400/2.8) has to be enlarged more which removes 1 stop of the difference...
Or think of it this way... an APS sensor (nikon) is a 1.5x factor (LxW), but the FL is a 2x factor (area).

Correction: I see my mistake... It only requires 1.5x in FL, not 2x. With the same composition DOF remains essentially the same (w/in 1/3 stop +/-). I.e. 400/2.8 APS vs 600/4 FF.
 
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The magnification is not the same, the image/image circle is larger on the FF sensor.
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The image circle would indeed be larger on the full frame sensor and the image, assuming the same composition, would be larger....this would result in a lower increase in magnification (when compared to the crop sensored capture) when viewing the image and result in the magnification being the same.

Bob
 
The image circle would indeed be larger on the full frame sensor and the image, assuming the same composition, would be larger....this would result in a lower increase in magnification (when compared to the crop sensored capture) when viewing the image and result in the magnification being the same.

Bob
Yes, I corrected my posts... but the DOF isn't greater, it is (essentially) the same... I was erroneously comparing a 2x lens magnification to a 1.5x crop magnification... probably because I tend to use a 2x w/ f/2.8 lenses more than any of the other options (and not always wisely).
 
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Once you keep the composition the same then you can forget all about crop factors, teleconverters and focal length......it's simply the aperture that controls the DoF for all reasonable limits.
 
Once you keep the composition the same then you can forget all about crop factors, teleconverters and focal length......it's simply the aperture that controls the DoF for all reasonable limits.
No, and that's because DOF has to do with physical display size; and the physically smaller sensor has to be enlarged more. It's the same ~ 1 stop APS difference that affects everything else (total light, etc)... i.e. "equivalence" requires the FF sensor to use a 50% longer FL at a ~1 stop smaller aperture setting compared to 1.5x APS (and the opposite is also true).
I realize that sounds wrong, but that's because FL has 2x the effect that Ap does... for example, starting from 400/2.8 a 50% increase in FL to 600/2.8 is ~ a 2 stop loss of DOF, stopping down one to 600/4 reduces that to ~1 stop less DOF than the original 400/2.8. But if the 400/2.8 image is on a smaller APS sensor it has to be enlarged more, which then negates/equalizes the DOF (w/in 1/3 stop)... i.e. an equivalent image w/ the same composition, DOF (physical display size), and total light.
 
The light per pixel consideration/debate is mostly irrelevant... it only relates to how an image appears to you on your display. It has little relevance as to how an image will appear to someone else on a different display or when printed. What actually matters is light per area recorded... ISO is not light/exposure.

.... However you want to analyse and define it all, when you are shooting in Manual-mode and controlling aperture-shutter-ISO, as you manually change the ISO you will actually see the target image's exposure change accordingly in a mirrorless viewfinder (Canon EOS R in my case). ISO might not BE exposure but it is one of the three fundamental ingredients in all photography which determine image exposure - Simples.
 
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ISO might not BE exposure but it is one of the three fundamental ingredients in all photography which determine image exposure - Simples.
Sure, it's "image brightness" much like the brightness control for your monitor... ISO invariance is/should be changing the way we think about and utilize ISO these days (camera dependent).
 
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