Illumination test for wideangle lenses?

[lars ake vinberg]

I want to test a center filter on a wideangle lens that the center filter is not specifically designed for. What is the best way to check for uneven illumination? Thanks & best regards, Åke

[bj_rn_nilsson]

The best way, in my opinion, is to photograph an evenly lit subject,

e.g. a gray card or something likewise. (Maybe something bigger, but

it should render an even gray negative.)<br>

Then you should give the negative a plus development, as this

enhances the effect of uneven illumination. Then on to the enlarger,

which I hope that you have already checked for uneven illumination.

(If not, adjust the lamp to the best of your abilites, and print the

full negative area, without a negative in the holder, on the hardest

contrast paper that you can find. You should adjust the exposure time

to print a middle gray, as any differenties are easier to spot that

way.)<br>

Now print the whole negative on a very high contrast paper. Adjust

the exposure time to get a middle gray value. Compare the result with

the one that you get without a negative in the enlarger.

[bill_marsh]

Schneider recently informed me that they never made a center filter for the 121 Super Angulon that I have. I

wonder if the one made for the 120 would work? Or any 82mm Heliopan? Or nothing?

 

<p>

 

Bill

[pete_caluori]

Greetings,

 

<p>

 

In addition to what Björn said, you can eliminate the effects of

enlarger error, by using a densitometer to read the density reading

on the negative/transparency. If you don't own a densitometer, you

may be able to have the density read at a commercial lab, university,

or even a hospital X-ray deparment.

 

<p>

 

Regards,

[michael_briggs2]

Focus the lens on infinity, then point the camera at a uniform object

which is uniformally illuminated. The reason for focusing on

infinity is that the unevenness will be worse at this distance because

the lens is closest to the film. Repointing at a closer object is

better because it is very hard to find a large distant object that is

uniform in brightness. The close object will be out-of-focus,

thereby smearing away any small-scale brightness variations. Be sure

to avoid shadowing part of the object being photographed.

 

<p>

 

Use the most contrasty material that you plan to use, e.g. B+W

developed to N+1, or N+2 if you do that, or color transparency.

 

<p>

 

I suggest taking a test photo with and without the center filter so

you can see what difference it makes. The manufacturers typically do

not design center filters to result in perfectly even illumination

because this would require long exposures and because it isn't

necessary. Following this approach, don't reject a center filter if

it doesn't yield perfectly even illumination--the goal is to reduce

the uneveness to an acceptable level. Real scences almost always have

some variation in brightness, and minor variations can be taken care

by burning in while enlarging.

 

<p>

 

When I did this, I judged the negatives by looking at them and

comparing the variation in density to the density range of a typical

negative. Going on to printing the negative would make it a full

end-to-end test. If you want to test your enlarger, you could make

another test negative in the same manner, using your longest focal

length taking lens.

[lars ake vinberg]

Thanks for all replies. Since I do not have access to a darkroom I

will have to study the film directly, or scan and evaluate in

Photoshop (which is the workflow I normally use). This probably means

that color transparency film is more appropriate.

 

<p>

 

It sounds like finding an evenly illuminated subject is the key. Here

is what I will do:

 

<p>

 

- Rig up a piece of white cardboard paper in sunlight, taped on a

window for flatness

 

<p>

 

- Set up the camera as close as possible to the paper without

acutally shadowing it,

 

<p>

 

- Focus on infinity,

 

<p>

 

- Expose for medium gray on color transparency film to also check for

color casts,

 

<p>

 

- Expose with and without center filter for comparison.

 

<p>

 

- Expose center as well as full shift, past image circle boundary if

possible.

 

<p>

 

What I want to avoid is a lens/filter combination that creates a

darker center, or a bright or dark halo because of dark spot size

mismatch. I can accept if the CF does not fully compensate. The CF in

question is for a 5.6/65 Super Angulon, and the lenses are a 5.6/47

Super Angulon XL and a 6.8/90 Grandagon MC, all with 67mm thread.

 

<p>

 

I will post my results in this thread.

 

<p>

 

Åke

[mike_mahoney]

I was going to suggest the Photoshop solution to you - simply scan a

transparency of an evenly illuminated solid color, and check the

various K values in PS. This would give you an accurate reading on

the existing light falloff, then do the same check with the filter in

place.