Lens Shading for the 85/1.2

[dk0]

First, I was able to answer my previous question on this forum, namely what did Contax actaully prescribe for its 85/1.2 50-year Anniversary lens. The official answer is the 77/86 Step-up ring and Metal Hood #3. This was verified by the service manuals from Contax.

Now for something more interesting. Another member of this forum made a claim about using Metal Hood #5 on the 85/1.2, which appeared rather dubious. Most other people claim that Hood #4 is the "best". Yet, I was puzzled by all of these claims which confront the work of the actual engineers at Zeiss who on two occassions produced the magnificent 85/1.2 lenses (the 50- and 60-year anniversary lenses), and yet insisted on a smaller hood size of type #3 in each case, even after ten years of re-consideration.

So, I decided to measure the variation of relative illuminance as a function of hood size myself.

Here is one image taken with the 85/1.2 shot wide-open against a uniformly lighted white-card using the 85/1.2 Anniversary Hood:

And here is another image taken wide-open with Hood #5:

I was able to quantify the light fall-off as relative illuminance with respect to the center. Here is a table of the results that show mean percentage of illumination from the edges to the corners (worst case). Higher numbers are better. In other words, in the perfect case of no light fall-off, the entry would be 100%.

For ex&le, the entry which reads 46.1 - 61.9 means that the mean variation of illuminance went from 61.9% at the edges to 46.1% at the corners (darkest).

As you can see, the Anniversary hood appears to be well thought out by the Zeiss engineers. Granted there are other issues like flare control. From this table you can see that deeper hoods such as the Metal Hood #4 are indeed a viable option for those who place more emphasis on this. However, you can see that there is just too much light fall-off when using Hood #5.

For those of you who still want more on lens hoods, check out http://www.vanwalree.com/optics/lenshood.html


David

 

[djg]

David, thanks for the excellent test and link.

DJ

 

[wang]

Well, I would like to ask those who has 85 1.2 to put on hood 5 and look at the viewfinder.

For me, I have S2b, AX and RX. When I put on my hood 5 there is no light fall off at the corners. I strongly doubt if the results produced by David are true.

David, what do you use to measure illuminance ? Where did you get those results ? Did you do it yourself or do you have a reference ?

 

[dk0]

Chi, I am a professional physicist, and am very capable of making scientific measurements. Indeed, I made these measurements myself. I do find it offensive that you claim my results are not true (even more so, when you claim that you can look through your viewfinder and instantly claim that my results as well as the engineering work from Zeiss are in error).

If you do not know the basics of measuring illuminance, then you can't possibly be qualified to comment on this at all.

 

[wang]

Well, perhaps what David found out could be right, but it is what you found in theory.
David, you can try yourself, the best is having an RTSIII with full view. You could see if you get dark corners.
For people who has Canon 1DsI or 1Dsll, they can easily test this on their cameras, to me there is no dark corners.
\Part of the reason that I insisted on this is because of the light pathways. This diagram showes the optical elements of 85 1.2. I draw the most marginal light pathway going thro' the lens.

Point A is the most peripheral point of the first frontal lens element. B is the same point of the second element. C belong to the third element. You can look at the this pathway directly with your eye as shown, the three points ABC coincide to become one point.
The photo showed the view of the eye so that ABC coincide to become one point. As you can see, the pathway remains intact with the hood 5 on.

Afterall, in terms of practical photography, the use of hood 5 on 85 1.2 does not produce dark corners in the viewfinder,negatives and prints. Most important of all, I have been shooting my 85 1.2 on hood 5 and never get a dark corners in my prints.

 

[dk0]

Chi, first please do not try to lecture on optics. You are only embarrassing yourself.

Light fall-off is a fact. The illuminance is always potentially higher in the center. The issue here is really whether you can or cannot visually detect a 10% difference in the light fall-off at the corners.

I have physcially demonstrated that there is an additional 10% loss of peripheral light resulting in images when using Hood #5. My gray images posted above were normalized so that the light fall-off can be seen in the corners. In other more realistic images, it may not make a perceptible difference to you, but it is there and it may make a difference to some people.

This will be my last posting on this thread, as there is nothing more of substance that can come from it. I simply tried to contribute something to those who might be interested in this from a purely technical viewpoint.

 

[gunteach]

Aside from the mild acrimony on this thread, probably prompted by a less than ideal use of the word "true"..I found this thread to be of great value in combination with the link David provided. What I gather from this thread is that light fall off does occur with lens hoods, to a greater or lesser degree, it's worse at wide-open than stopped down, in some cases it really isn't visible and flare reduction may be the priority anyway. Hope I got the lessons of this thread right.

I for one always thought a lens hood wasn't really necessary and that size and shape did not matter on anything other than the widest angle lenses. Now I know better. Thanks David.

 

[jsmisc]

Yes, thanks to all and particularly David for his scientific explanation and for the useful link. I have found it interesting too. I invariably use a lens hood but I had no idea that the difference in shapes could make such a difference. I had always felt that the tulip type would be less efficient and that the rectangle shape was strange. It just shows how wrong you can be.
John

 

[wang]

David, on your posting 11, you said I am embarrassing myself. I don't feel that I am embarrassing myself at all. Can you tell me if I have written anything wrong ? I am presenting to everyone that hood 5 is not in the way of the light travelling pathway, this is a decent fact. Can anyone tell me if this is embarrassing ?

David, on your posting 11, you said I tried to lecture on optics. Let me tell you that I did not tried to lecture on optics, but I am stating the truth in optics. Can you tell me if I have written anything which is wrong. Do I have to try ? I am simply stating the correct fact in optics.

Perhaps, the problem is the attitude of David of trying to make other people feel embarrassed. Sorry, I really don't feel embarrassing at all. I am presenting a very decent work.

David, if you are a professional physicist, why don't you divert your effort of trying to make other people feel embarrassing to explaining why there is a 10 % loss of peripheral light when the light travelling pathway is unaffected ? This is something I really wish to know.

 

[wang]

What I feel embarrassing is the 10% difference was showed up with darker corners.