Using NON DIGITAL lens on DIGITAL SLR

[sean_buerger]

Hello Camels!

To save some money I use two types of adapters on my SIGMA SD10. One is for M42 lens type the other one is for NIKKOR lens type. The link is purely mechanical and does not extend to electronic controls (AF motor, aperture control). This reduces obviously the functionality of the camera - lens combo. It does give me the opportunity to use glass that otherwise would collect dust only

When doing a rather comprehensive test with some prime lenses from Nikon, I noted a discoloration in the centre of the picture, when stopping down. I suppose that is due to reflection of the aperture blades or whatever. Fact is that the lens was not designed for reflecting CMOS sensors, rather than film.

This is what I get. Anyone note something like this before...

[sean_buerger]

Oops. Forgot the picture

[metalstorm]

Not so sure about whether a lens is designed for CMOS or not. Maybe some coatings help it. Most of the FF lens are/were designed with film in mind and they seem to work rather well on most digitals crops.

[jfrancho]

You say "stopping down," - how far down? It could be diffraction, which, for Canon sensors, usually manifests as softness, starting at f/11. I have no idea what the limit is for Sigma. There are quite a few vintage lens enthusiasts I know that use some very old glass on digital SLR cameras with excellent results. Do some experimenting with different apertures, and let know what the results are.

[bill8131]

Sean,

For what it's worth, I've been shooting straight digital for several years and don't own a "digital" lens, I've never seen anything like that.

Now, I don't don't want to start a verbal war with the Nikon gang, but, if this is only happpening with the Nikon lenses, that would explain (posssibly) why Nikon went so heavy into the "digital" lens series, not just for the ratiio difference? Just wondering.

And yes, my gear is all Canon. *I hope someone has the correct answer! it's an interesting one, that I'm sure will have a lot of opinions!

Bill

[sean_buerger]

The picture shown is stopped down to 16 (as shown in the filename).

On my 1.2/55 the 'purple spot' effect becomes visible at f=5.6 and get's more obvious from there onwards. On the 3.5/28 I can see more or less the same thing (with less intensity) starting at f=11.

However those two, are the only ones having this effect so far.

What SIGMA claims on their website, is that they have specific coating on the lens (near side camera) to prevent harmful reflections from the imager. I don't know, whether the aperture blades are a factor here.

Cheerio. Sean.

[sean_buerger]

Ok. Let me show me the test results.

I would like to add that I LUV NIKON lenses. The 1.2/55 is crisply sharp at f=2.8! The spot issue was seen also on other sets (one indoor and another outdoor at infinity). Only when stopping down, I get this...

[sean_buerger]

And more aperture settings...

[sean_buerger]

As I'm stopping down (see filename), you can see the centre of the picture gradually turn colder until the purple spot comes out.

I should add that the Nikkor 1.2/55 is really old (probably my age, haha)

Cheers.

[jfrancho]

My very rudimentary understanding of this, is that at higher aperture settings (small aperture, "stopped down") the light is too unidirectional to properly illuminate each photosite on the sensor, and therefore results in softness and discoloration. How the so called "coatings" work to mitigate that, I have no idea. I, like Bill above, use Canon, and exactly none of my lenses are "digital," or designated EF-S. In fact, I'm pretty sure my 35 f/2 is closer to 20 years old than 10, which automatically predates the digitalera for consumers.

[sean_buerger]

I might like to add that there is no softness (you cannot see that from the scaled down images). It's discoloration only.

Sean.

[Andy_T]

Sean, I don't have a satisfactory answer to your question other than some ideas, but I have been using some third party lenses (M42, Rollei mount, Contax mount) and have not seen such behaviour.

Take a look at this manual lens forum: http://oomz.net/mf/

As to my ideas ..
- could it be some kind of lens flare?
- could it be some kind of 'hotspot' for UV light? Maybe put on an UV filter and see if it changes anything.


Best regards,
Andy

[sean_buerger]

I think it could be lens flare. I did not try the UV Filter yet, but I can test this (on the weekend). However I did have the same occurence on an indoor scene, which should not be high UV.

Thanks for your ideas. Sean.

[scoundrel1728]

A few questions:

1. Do you normally use a protective filter over your lenses? If so, is it antireflection coated?

2. Your series of samples is backlighted, with the sky and possibly the sun shining brightly onto the front element of the lens. When you notice this blue spot, do you have a relatively strong source of light outside the image area?

3. Do you normally use a lens hood with these particular lenses?

[blumesan]

Doesn't seem to me that lens flare would worsen as one stopped down. As someone else has suggested, it would be interesting to see if this phenomenon occurs regardless of light direction. What happens when you shoot a blank white wall? Also would be interesting to see what happens if you used these lenses on a film camera. It surely is bizzare.

[scoundrel1728]

On my Canon 350D, the imager assembly doesn't reflect a lot of light but it does have a mirrorlike finish. I am wondering if one of the surfaces in these particular lenses is acting like a concave mirror and is focusing an image of the aperture opening back onto the imager.

This would explain a lot of things, such as:
1. The central spot gets smaller as the aperture opening gets smaller.

2. The apparent brightness of this reflection increases as the diaphram is stopped down. This requires some further explanation. The intensity of this image would not change as the diaphragm is stopped down, though its area would, and the intensity of the expected image would of course decrease. The exposure time would of course increase with the decreasing intensity of the expected image, resulting in a proportionate increase in the contribution of the diaphragm image to the whole as the aperture is reduced.

3. The blue-violet color of the resultant spot because the intensity of the offending lens surface would be fairly strongly colored, especially if the peak spectral intensity is at the extreme red or violet end of the spectrum.

4. The fact that this spot is not noticeable in the viewfinder.

5. The fact that this phenomenon was never noticed with film because film produces a diffuse rather than a specular reflection.

6. The fact that the phenomenon is lens-dependent.

7. Central hot spots simillar to this are sometimes reported when shooting infrared or ultraviolet with digital. This would be expected because the antireflection coatings are less effective outside the visible band and such reflections would be more likely to show up.

If I am correct, then:

1. The central spot's diameter will be approximately proportional to the aperture's diameter.

2. If the shutter speed, ISO rating, lighting, and subject are held constant, then the intensity of the spot will also be (approximately) constant regardless of the aperture. This would be easier to measure if you shot an object like a checkerboard with intensely black areas interspersed with your image. Something like this chart I originally intended for measuring barrel distortion and lateral chromatic aberration:



For this purpose, the regularity of the black squares isn't especially important but their blackness is. You might try pasting bits of black photo paper or velvet onto a white background. This will make measuring the intensity of the blue spot a lot easier. You may get a higher reading at larger diaphragm openings because of ghosting and more conventional lens flare.

3. You should get a similar result when mounting this lens onto another camera model, though the overall brightness of the reflection may be different. If so, you should be able to see the difference when you look at the imagers of both cameras set for a long time exposure or an imager clean.

4. This particular lens would probably be a lousy performer if you shot digital ultraviolet images.

[sean_buerger]

I thought I'd comment on the suggestions...

I included a picture taken indoors. The light is coming from a window at the side. The camera is fairly far away from the window and there is a corner in the wall, so there's practically minimal direct light falling onto the camera. Still the centre discoloration is visible, although less obvious.

All shots with this lens were done without a lens hood or skylight (uv) filter. I will do a test with a Hoya HMC Sky filter on the weekend, to see whether there is any effect.

I've checked up a bit on the history of my lens, which is apparently a type II. Which makes it quite old and not all lens elements do have amber coating. So it's not a multicoated lens throughout. I've used it before on my Nikon F2 and it never showed this kind of discoloration, although it's a bit prone to lens flare.

It is correct to say that the discoloration does not appear visibly in the view finder, when stopping down. I would have noticed that, as I need to stop down before shooting. After all your valuable comments, I presume it's the aperture blades interacting with the lens design and the reflective surface of the imager.

So, if you're trying old glass on your new digital toy, it might be worth testing it systematically over the whole aperture range, to avoid surprises!

Let me follow up with you on the skylight filter next week.
Cheers and thanks to all.
Sean