Vignetting Analysis - 47mm to 1200mm Lenses

[genotypewriter]

Hi everyone,

I computed the relative light gathering differences for typical LF lens focal lengths from 47mm to 1200mm, based on the cosine fourth formula (assuming non-tele and non-retro), and made the following illustrations:

http://www.flickr.com/photos/genotypewriter/5774394382

There's a bit of a write up on what the above info is and isn't in the page above which might be good to read to understand the intentions.

Would like to hear people's thoughts :)

G

[vinny]

My head hurts now. Every make of lens in the same focal length is different, as you said below your graph. I'm not sure of the practical value of this information.

[timparkin]

My head hurts now. Every make of lens in the same focal length is different, as you said below your graph. I'm not sure of the practical value of this information.

Thanks for this - it confirms a symptom that a colleague has observed through experience.

Tim

[Dan Fromm]

Example practical use of values in table: You have a 150mm lens and you're considering on getting a 65mm lens (1/2 field of view) to gain 1 stop in the amount of light needed (at same f-number) to keep the camera steady. While this benefit is valid, you will also lose slightly over 2/3 stops of overall frame brightness due to the increased vignetting in the wider lens. So the final theoretical gain would be only 1/3 stop.

WTF? 75 mm, no 65 mm, gives double the field of view of a 150 mm if both lenses cover the same format. Check your arithmetic. Proofread with care.

And, does anyone here change focal length to get more (or less) illumination? I certainly don't. The text I quoted seems very silly.

OP, are you aware that some, not all, modern wide angle lenses beat cos^4?

[GPS]

Of no practical use I can think of. If you're not happy with light fall of your lens use a centre filter.

[Leigh]

I'm inclined to use the manufacturers' data rather than yours.

Sorry.

- Leigh

[genotypewriter]

Hi folks,

Science doesn't have one magic equation that describes the whole universe yet but that hasn't stopped mankind from trying to understand it from the different things we already understand. Similarly, the purpose of this vignetting analysis is to understand the standard "cosine fourth" form of vignetting that is common to all lenses of these focal lengths, as stated, as long as the lenses are not of retrofocus or telephoto designs.

Design and build specific vignetting takes place in addition to the numbers I've given. So while the final relative illumination curve can take a different shape for a particular lens, the cosine fourth light loss constitutes to it as well.

WTF? 75 mm, no 65 mm, gives double the field of view of a 150 mm if both lenses cover the same format. Check your arithmetic. Proofread with care.

Dan,
Not sure why you think arithmetic alone is sufficient. We need trigonometry to calculate the field of view:

http://en.wikipedia.org/wiki/Angle_of_view#Example

You might want to plug in the numbers.

And, does anyone here change focal length to get more (or less) illumination? I certainly don't. The text I quoted seems very silly.

Sure, you and the people you know may not but the fact remains true. Under practical conditions, especially when stability is a concern, wider lenses can manage longer exposures than longer lenses of the same f number (for the same framing) giving the sense of being able to gather more light. At the same time, the cosine fourth vignetting takes its toll in the total amount of light gathered.

That is just one situation where the numbers I gave may be useful in a lens buying decision. I'm not saying that just because the table or the purpose of the table is not understood, one can't take good photos.

OP, are you aware that some, not all, modern wide angle lenses beat cos^4?

These modern WA lenses that you refer to are retrofocus designs. Which is why I said in many places that these numbers can't be applied (at least directly) to retrofocus and also telephoto lens designs.

Also "beat" is a funny thing to say here because the absolute light fall off itself is not overcome. Retrofocus lenses need a longer extension from the imaging plane than an ordinary lens for the same focus distance. This makes the retrofocus WA lens have the cosine fourth vignetting of a longer ordinary lens, which would be less than that of a ordinary WA lens.

Like I said, the cosine fourth light fall off takes place after the light leaves the rear of the lens and before it hits the imaging plane. The absolute loss can't be overcome.

Of no practical use I can think of. If you're not happy with light fall of your lens use a centre filter.

Agree that a center filter is good choice. A long lens can't replace a wide one all the time. But like I said to Dan, when the total amount of light gathered is of concern, the table can provide some insight.

Thanks everyone for the replies!

GTW

[Dan Fromm]

Not sure why you think arithmetic alone is sufficient. We need trigonometry to calculate the field of view:

Thanks for the correction. This is what I get for posting before the coffee has kicked in.

These modern WA lenses that you refer to are retrofocus designs.

Biogons retrofocus? Are you sure? With them illumination on film falls off with roughly cos^3. And with them back focus is roughly half focal length.

I think you're in 35 mm SLR territory and not thinking about the differences between lenses for them and lenses for LF cameras. There are very few retrofocus lenses for formats larger than 6x7. Name a few, please.

Under practical conditions, especially when stability is a concern, wider lenses can manage longer exposures than longer lenses of the same f number (for the same framing) giving the sense of being able to gather more light.

Explain further. I don't see how shooting at lower magnification and enlarging more in printing will reduce motion blur in the final print.

[genotypewriter]

Thanks for the correction. This is what I get for posting before the coffee has kicked in.

Always a pleasure :)

Biogons retrofocus? Are you sure? With them illumination on film falls off with roughly cos^3. And with them back focus is roughly half focal length.

I think you're in 35 mm SLR territory and not thinking about the differences between lenses for them and lenses for LF cameras. There are very few retrofocus lenses for formats larger than 6x7. Name a few, please.

As I said many times, Dan, this is a simplified analysis and is not meant to be taken as the gospel of vignetting :) Yes, by optimising and increasing the complexity of the computations (e.g. incorporation BFL, etc. and other parameters) the figures can be made more accurate for some lenses, at the same time it can become less accurate for others. You might also want to read this:

http://en.wikipedia.org/wiki/No_free...d_optimization

Explain further. I don't see how shooting at lower magnification and enlarging more in printing will reduce motion blur in the final print.

I don't think I ever said anything like that. Crosstalk from a different discussion perhaps? :)

GTW

[Dan Fromm]

GTW, you wrote this:

Under practical conditions, especially when stability is a concern, wider lenses can manage longer exposures than longer lenses of the same f number (for the same framing) giving the sense of being able to gather more light.

Please explain.

And while you're at it, since you're posting in an LF forum please give a list of retrofocus lenses for LF.