Depth of Field (DOF) and Teleconverters (TCs)???

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Way back, the original question was if adding a teleconverter changes depth of field. Based on simple optics, I had posted a response that it really should not. (I tend to use simple optic systems to let me understand basic concepts and then just try to learn how my lenses work out by using them and seeing what things look like - anything more is just too much math!).

Calculating depth of field, for me, really can’t be as precise as the online calculators suggest due to the subjective assessments of what is acceptable sharp focus, how is the out of focus area rendered, what does the transition from acceptable focus to blur look like (abrupt or gradual), etc. The calculators can be fun to play with to get an idea of what one can expect.

Just for fun I made this set of images. I used a 70-200 zoom at the 200mm setting (which is listed as 190mm in the metadata) and kept the aperture at f/5.6 (i.e. not physically changing the aperture setting on the lens for both images). The lens is on a tripod and is focused on the thumb pick (in all of it’s “clown barf” colors!). The first image is without the teleconverter. I then added a 1.4x teleconverter but kept the lens at the same position, same focus point, same set f-stop and took the second picture. Then for the third image I visited Photoshop to align the two pictures (no TC and with TC) so I could crop the first (no TC) to the field of view of the second (with TC). (poorly lit office so used a high ISO).

To my eye, looking at the images at the same size on screen and at the same distance (and looking at a print of all three together) the actual narrow area of what I think is acceptable sharp focus is pretty much the same but there is a bit of variation. The “19” mark seems similar in all but the “17” mark is not as sharp in the teleconverter image than in the non-teleconverter one (which also is not fully sharp at "17").

What is interesting to me is the more gradual changes seen when the teleconverter is not in place and the more abrupt transition to blur and very different rendering of that blur when the teleconverter is used.

So, if one defines depth of field as the area in front of and behind the plane of focus that is essentially as sharp as the plane of focus then I do not see, in my small test, significant differences. However, if one looks at the overall transition from sharp to sort of sharp to blur then the apparent depth of field when using a teleconverter does appear to be shallower. Interesting!

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This is a great post in this thread, because it shows that actual use reveals things of importance that pure theoretics may easily miss. Adding a TC may not do much in the way of altering DOF assuming you intend to crop images to a comparable FOV. So say you are shooting birds at 600mm f4 and cannot get closer whilst not being able to fill the frame as much as you would like, you may put on a 1.4TC to get to 840mm f5.6 to fill the frame as much as you like, ór you may continue shooting at 600mm f4 and use the high resolution FF sensor to crop to the desired level of filling the frame in post. The results regarding DOF would be very similar. But a TC causes the OOF rendering to change, and that is something you may like or not like, but the difference is there. Anyone with good eyes having used a 1.4TC a lot, will start to recognize the characteristics of the OOF rendering compared to cropping the bare lens.

Another thing is, that desired DOF may be very different in different scenarios even with the same lens. Personally, I feel that at 600mm, a f4 aperture is not desireable when subjects are at close to medium distance and at fair distance from their backgrounds, it results in an alienating, overly dreamy look that is lost on me, but when subjects are a bit further away and they have their backgrounds fairly close behind them, the f4 aperture at 600mm can be a godsend. Hence I don't just set a 600mm lens at f4 all the time and forget about it, but adjust the aperture according to the situation, to my liking of course. When however you decide to shoot a smaller sensor size, aps-c or m4/3, and/or use a different (slower) lens at a certain focal length, and you have calculated that DOF will be desireable compared to an "equivalent" full frame set-up, you forget that you loose the flexibility that such a set-up provides. You may end up locked in by such a system.

Theory should always take a back seat to just using equipment in te field and see what look it produces and what works for you.
 
Or we could hold in that DOF preview button on the front of the camera.
In an increasingly mirrorless camera body environment the depth of field preview button is redundant at apertures smaller than about f5.6 - as the viewfinder/monitor shows the actual depth of field being recorded on the sensor as the apertures is changed between wide open and about f5.6 - usually with distinctly less viewfinder dimming than when using a DSLR viewfinder.

For those yet to embrace ML a camera body button can usually be programmed to act as a conventional dof preview button to show depth of field at apertures smaller than f5.6 - again usually with less viewfinder darkening than with a DSLR

Many photographers using "Canikony" systems are probably not overly concerned about the maths behind behind how depth of field in the image is achieved.

Query - do any ML systems display depth of field at smaller apertures than f5.6 without using the equivalent of a conventional dog preview button?
 
In an increasingly mirrorless camera body environment the depth of field preview button is redundant at apertures smaller than about f5.6 - as the viewfinder/monitor shows the actual depth of field being recorded on the sensor as the apertures is changed between wide open and about f5.6 - usually with distinctly less viewfinder dimming than when using a DSLR viewfinder.

For those yet to embrace ML a camera body button can usually be programmed to act as a conventional dof preview button to show depth of field at apertures smaller than f5.6 - again usually with less viewfinder darkening than with a DSLR

Many photographers using "Canikony" systems are probably not overly concerned about the maths behind behind how depth of field in the image is achieved.

Query - do any ML systems display depth of field at smaller apertures than f5.6 without using the equivalent of a conventional dog preview button?

My Canon R5 does not show DOF at all unless I push the preview, I don't think. I don't know if it does the 5.6 thing like Nikon does. If not I'm glad it doesn't as I'd rather manual focus wide open on the exact plane of focus rather than fight the DOF preview.
 
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