[TDvN57]

I was wondering if the depth of field between two similar focal lengths for different formats would give different or similar DoF when used on the same camera. My experiment is limited and definately not scientific, but I tried to get an honest guideline for myself. So here is the setup:

Camera: 645z on a sturdy tripod.
Lens 1: M* P67 400mm f/4 lens with Pentax adapter to 645.
Lens 2: FA 645 400mm f/5.6

"Studio": Dining room table with house light overhead and house style background objects.

Camera settings:
ISO 100
Lens setting f/5.6 for shallow DoF and F/8 for deeper DoF
Shutter speed: P67 lens 2s for both pictures; P645 2s for f/5.6 and 4s for f/8. Dont ask me why the last pic was 4s, it is just the way it happened, I felt it was under exposed.

Object:
Pentax KP with 28-70mm lens.

Methodology:
Take two pictures with each lens, one at f/5.6 and one at f/8. When I changed from the 67 lens to the 645 lens I roughly measured the distance from the front of the lens to the object and moved the tripod forward to match the distance with the 67 lens distance.
Post editing is minimal and no cropping, but with WB set the same for all pictures. The distance from the camera to the object is slightly more than at the minimum focussing distance.
Both lenses were focussed manually on the name plate of the 28-70mm lens, using magnified live view.

The 67 lens pictures are at the top and the 645 lens pictures are at the bottom.

Observations:
1. The KP needs a good cleaning.

2. The magnification between the two lenses are not similar and the 67 lens magnifies slightly more despite being at the same distance. A rough measure in Photoshop indicates a difference of about 5%.

3. Comparing the blurring of the backgrounds between the 67 and 645 lenses, the 67 lens blurring is much more creamy.


Conclusion? Well I thought I would ask you guys to help draw some conclusions, because sometimes the obvious is incorrect.

Attached Images

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Last edited by TDvN57; 02-22-2023 at 01:37 PM.

[JensE]

Take two pictures with each lens, one at f/5.6 and one at f/8. When I changed from the 67 lens to the 645 lens I roughly measured the distance from the front of the lens to the object and moved the tripod forward to match the distance with the 67 lens distance.

Distance is normally measured from the sensor plane, so your difference is even a bit bigger. The difference in magnification can be due to the focusing mechanism at 'close' distances, it may be smaller at infinity - the internal focus mechanisms may shorten the focal lengths in different ways. The actual focal length however often is slightly different than the rounded nominal one in the lens name.

[pschlute]

Dont ask me why the last pic was 4s, it is just the way it happened

All four images look like the same exposure. Something is not right with your setup if you say some are at f5.6 and some are at f8

Once you put an adapter on the camera/lens all bets are off. What does the manual for the adapter say about change in focal length ?

[TDvN57]

All four images look like the same exposure. Something is not right with your setup if you say some are at f5.6 and some are at f8

Once you put an adapter on the camera/lens all bets are off. What does the manual for the adapter say about change in focal length ?

The adapter does not have any glass elements, it is merely a spacer that puts the 67 lens at the same flange distance it was designed to. Optics and lens behaviour should not be affected.

[pschlute]

Ah. I will bow out in that case. Hopefully someone with experience of this can advise you.

I was thinking it would act like a TC but that is not the case it seems.

[TDvN57]

Here is an +/- 80% crop which shows a bit more details, I didn't realize the forum pictures dropped that much detail.

[RICHARD L.]

Hello Theuns,


I was thinking about finding a nice copy of the P67 M* 400 mm f/4 tele but it costs "mucho dinero" even used. I have a 645 400 mm f/5.6 and your pictures show its performance is quite honorable compared to the P67's. I don't use these focal lengths much so I won't spend over 2000 $ for the big white lens.

Regards

[marcusBMG]

Um, not quite sure what the question is here but couple of points:
- it's the distance to the sensor plane that counts, not the front of the lens.*
- On the same sensor (camera) all lenses of the same focal length will show the same depth of field, other aspects being constant (f-stop, camera-subject distance etc). Larger format lenses like P67 will project a larger image circle, of which only a smaller part is utilised by a smaller sensor, but that's irrelevant to DoF.
*actually thinking about this I am not so sure, in principle that's right, in practice it might be from the optical centre of the lens or st, in any case we're only talking a few cm so the difference due to longer/shorter lenses is relatively negligible.

[UncleVanya]

As several indicated, at the same focal length and aperture depth of field is the same. Sensor size is like windows size in your house. Think of a large format lens like opening the curtains wider - if the window (sensor) isn’t bigger the extra image parts just are wasted with no effect on the image. The center of the image doesn’t change at all. The reason depth of field changes with sensor size is that you use different distances or focal lengths for the same framing. It’s not inherent to the sensor itself, just the way lenses are used with it. The moment you hold focal lens length constant, and crop to the same size, the depth of field is not impacted.

Bokeh is another thing altogether. The quality of the out of focus areas aren’t just from depth of field. I think that’s what you are seeing.

In addition 400mm may be 383 and 405 it’s hard to directly compare without carefully measuring the distances etc.

Moral of the story: depth of field isn’t changed, but one of these lenses is nicer to look at the background with.

[Alex645]

If you want to see the theoretical DOF based on format and focal lens with hyperfocal distances, here's an online table:

Depth of Field Table

[Ian Stuart Forsyth]

The reason depth of field changes with sensor size is that you use different distances or focal lengths for the same framing. It’s not inherent to the sensor itself, just the way lenses are used with it. The moment you hold focal lens length constant, and crop to the same size, the depth of field is not impacted.

There is no DOF until we view it


And when we view it there is a different amount of enlargement that will cause different DOF from the same lens

If we ignore how we view the final image ( the same output size)we can come up with a number of DOF for the same lens at the same working distance and the same f stop on the same format .

If we ignore how we are going to view the image I can easily say that my FF camera using a 50mm at F1.8 shot at 8m away will have the same DOF as the same lens shot at F4 at the same distance.

[UncleVanya]

There is no DOF until we view it


And when we view it there is a different amount of enlargement that will cause different DOF from the same lens

If we ignore how we view the final image ( the same output size)we can come up with a number of DOF for the same lens at the same working distance and the same f stop on the same format .

If we ignore how we are going to view the image I can easily say that my FF camera using a 50mm at F1.8 shot at 8m away will have the same DOF as the same lens shot at F4 at the same distance.

I think you totally missed a key point. The two lenses were shot are being compared on the same sensor and while I oversimplified it - the point that should be made is that the lens format makes no contribution to depth of field at all. And the sensor size (including pixel sizes) contributes only a small amount compared to the effects of using different focal lengths to cover the same angles of view.

[Ed Hurst]

Yes, the differences you are seeing are largely due to different bokeh characteristics of the lenses

[TDvN57]

I did a few more pictures this morning, this time I upped my game a little bit. This time I used a measuring tape to measure the DoF, or rather not measure by get an eye-ball impression on it. I also measured the distance from the camera focal plane indicator on the outside of the camera to the start of the measuring tape to be 2.87m, thus the actual distance to the center point of focus was 2.87 + 0.15 = 3.02. Just clearing the minimum focussing distance for the 645 FA 400mm.

Interestingly the magnification of the 645 lens is somewhat less than the 67 lens, similar to what I saw in the first round and expanded upon by others in the comments.

I gave my best guess to the DoF on the tape and it looks like both are at the same 2cm @ f/5.6 and 3cm @ f/8. Thus sort-of confirming my understanding of DoF and also mentioned by many in their comments that all things being equal, the size of the circle of light on its own does not affect DoF, but rather the size of the sensor/film frame.

Considering the bokeh seen on the first round I am seriously considering experimenting with these lenses for close-up portraits. The problem with flatness look of a long lens can easily be countered in ACR/Lightroom in the optics section with the distortion slider.

[Alex645]

I was wondering if the depth of field between two similar focal lengths for different formats would give different or similar DoF when used on the same camera.
Conclusion? Well I thought I would ask you guys to help draw some conclusions, because sometimes the obvious is incorrect.

With DOF, it's splitting hairs. It's going to be more similar than different.
The 67 creates a larger image circle, and thus for the 645 sensor, you're getting more of the sweet spot on the 67 than the 645.

FA 645 400mm angle of view is 7.9°. 7 elements, 6 groups, 8 aperture blades. 1250g.
M67 400mm angle of view is 12.6°. 9 elements, 9 groups, 9 aperture blades. 3700g.

Of course the angle of view on a 645D is going to be nearly identical. The optic design will create other minor differences, and the bokeh will become more apparent closed down with diffraction too.

In real world usage, outdoors shooting wildlife or landscapes, I'd be more interested in seeing differences in chromatic aberration. And the M67 would have to be clearly superior for me to want to haul a lens triple the weight to the 645.