[dms]

This made me curious. So I took 2 shots with no hood and 2 with the DA40 hood all @ same aperture, shutter speed, ISO and controlled light condition. Again using a white wall as subject (metered @ 18% gray) and 2 different focus distances. I can not discern with my (aging) eye any difference in the shots. Certainly not a full stop. I do believe the DA 40 hood has a 30mm opening though rather than a 28mm opening. So it is somewhat closer to the theoretical 3.57. Or 100/30 = 3.33... Interesting.

Generally** the vignetting is worst when focussed at infinity, and at the smallest aperture. So w/ a prime lens shoot a light wall with focus distance set to infinity (or anyway a large distance), and f/22 or the smallest aperture you could use.
____
** zoom lenses could have odd FL changes w/ distance.

[bdery]

In the review of the DFA100 macro here at the forum, a step down ring 49mm to 28mm is suggested as a substitute for the lens hood. Where can I obtain such a device? So far I have not found one at the usual sources.

When I wrote that review, this type of step-down ring was common. Nowadays indeed, it seems to have become rare. I can't explain why, but a combo of two steps will work.

Hmmm.... 28mm seems a bit small and will certainly cause vignetting.

I think the lens is already slightly recessed in the DFA 100mm and it has good lens coatings, so you might not need 28mm. You have to be careful if the step down ring goes too far, it will cause vignetting.

Please, friends, read the review and the tests before commenting on this. It does not vignette (on APS-C, which is how the test was made as the K-1 did not exist at the time).

100mm / 28mm = 3.57 so you will be losing almost a full stop.

The smallest opening you should use is at least 36mm.

Your calculations work for FF (which is why, about a year ago, I posted a follow-up saying that a 49-37 step-down ring should be used for FF). On APS-C 28mm is ideal.

[UncleVanya]

Your calculations work for FF (which is why, about a year ago, I posted a follow-up saying that a 49-37 step-down ring should be used for FF). On APS-C 28mm is ideal.

The vignetting is undoubtedly better on APSC. However, lens focal length and aperture are not related to the size of the frame captured. It is possible that the apparent size of the hole is larger based on the optics of the lens but I suspect it is more likely just a small change in the aperture that is hard to pick up in side by side comparisons.

[bdery]

However, lens focal length and aperture are not related to the size of the frame captured. I

I never claimed that

It is possible that the apparent size of the hole is larger based on the optics of the lens but I suspect it is more likely just a small change in the aperture that is hard to pick up in side by side comparisons.

I'm not sure I understand this, and suspect we're not talking about the same thing.

To understand why a 28mm hole is enough on APS-C, think about the DA21mm's hood, which is essentially a rectangle hole. The image of that hole on the focal plane needs to be larger than the sensor or it will be visible. If the sensor is smaller, the hole can also be smaller and still have no effect on the image. That's why for APS-C a smaller diameter works.

With a cylindrical hood (like the one provided with the 100mm macro) the principle is the same, the hole being farther away from the lens's front and thus being bigger.

[photoptimist]

I never claimed that



I'm not sure I understand this, and suspect we're not talking about the same thing.

To understand why a 28mm hole is enough on APS-C, think about the DA21mm's hood, which is essentially a rectangle hole. The image of that hole on the focal plane needs to be larger than the sensor or it will be visible. If the sensor is smaller, the hole can also be smaller and still have no effect on the image. That's why for APS-C a smaller diameter works.

With a cylindrical hood (like the one provided with the 100mm macro) the principle is the same, the hole being farther away from the lens's front and thus being bigger.

Although the intuition that smaller format sensors can have smaller/tighter hoods is true, it the hood size is smaller than the maximum aperture, the hood will cut into the light gathered by the lens even in the very center of the sensor. Even on a tiny sensor camera like the Q, the 100 DFA needs at least at least a 35mm hole. For APS-C the hole needs to be a bit bigger than that.

[bdery]

it the hood size is smaller than the maximum aperture, the hood will cut into the light gathered by the lens even in the very center of the sensor.

Correct.

Even on a tiny sensor camera like the Q, the 100 DFA needs at least at least a 35mm hole. For APS-C the hole needs to be a bit bigger than that.

Incorrect. As I wrote earlier, this was tested in the review (and by many users also). As one writer used to say, RAFO (read and find out)