How does barrel lens shutter placement affect the image?

[AidanAvery]

Last night I ordered a packard shutter for my barrel lens. Researching different fashions of mounting the packard shutter, I found a post that described how mounting the shutter on the front of the lens or the back of the lens actually impacts the incoming image differently.

Specifically, if the shutter is placed behind the lens, the image has already been formed while passing through the lens before it reaches the shutter. So, effectively, as the shutter opens and closes it simply reveals the image to the film plane.

On the other hand, if the shutter is placed in front of the lens, it impacts the incoming light prior to the formation of the image. In this case, the shutter acts like a variable aperture that quickly widens and then closes.

This second point kind of blew my mind because, as we all know, different apertures create different optic qualities.

I had been planning on mounting my packard on the front of my barrel lens (it won't fit behind, in any case); however the second point started me wondering about how this might impact the images.

So, a question for those of you with more experience/knowledge of barrel lenses, front-mounting shutters, optics, etc... At the packard's 1/25 speed, will front-mounting the shutter noticeably impact with final image at all? will it change with the depth of field since it is acting as a variable aperture instead of a static, wide-open one? Will it have other effects on the image that I'm not considering? Or, will it open and close so quickly that my final image will still look as if the lens was shot "wide open," at 1/25 shutter speed?

I am particularly curious because my barrel lens is a petzval and I don't want the shutter to reduce its "petzval-y-ness"

EDIT:
Thanks to @reddesert for clarifying with the response below:

It does not matter. The main issue is to make sure that when fully open, the shutter does not vignette your lens.

First, the amount of time that the shutter is opening and closing is a small fraction of the total time.

Second, the shutter either in front of or behind the lens is not exactly the same, optically, as the aperture inside the lens. The aperture inside the lens forms the pupil or aperture stop that governs what rays can pass through the lens. The pupil is maximally out of focus. For optics reasons that are too complex to explain quickly, the shape of the pupil determines the diffraction pattern of the lens, which is why for ex a five bladed aperture will give you a different number of light spikes from a point source, or shapes of out of focus blur (bokeh) compared to a four bladed aperture. However, if you stick a pentagonal hole in front of or behind your lens (like using a vignetter in a matte box), you'll get a pentagonal vignette.

There is a lot of optics misinformation or half-information on the internet, and the idea that the shutter has a different effect depending on whether the lens has formed the image already might fall into that category. What matters is whether the shutter is vignetting the lens. Vignetting can change with front or back placement, depending on lens size and angle of view.

[Tin Can]

Test for yourself

Tell us the lens and camera

This rare Deardorff has 8"OD ID 4.5

I wish it was new Packard

Xx by TIN CAN COLLEGE, on Flickr

[Kevin Crisp]

I really don't think it will matter at all. An image forming "before" or "after" doesn't make a practical difference because the light is moving at....the speed of light. For all intents and purposes, things are happening at the same time. I've front mounted a number of lenses (big repro clarons, hanging out front of Compur #2) and I've used packards behind barrel lenses. I don't think any of these combinations altered the lenses' personalities. If you start moving around where the aperture of a barrel lens is relative to the front and back elements, I think you might seem some difference, but I haven't tried that beyond remounting lenses in shutters and finding that small thousandths of an inch difference make no difference.

Lots of people have used hats and caps for shutters.

[AidanAvery]

I really don't think it will matter at all. An image forming "before" or "after" doesn't make a practical difference because the light is moving at....the speed of light. For all intents and purposes, things are happening at the same time. I've front mounted a number of lenses (big repro clarons, hanging out front of Compur #2) and I've used packards behind barrel lenses. I don't think any of these combinations altered the lenses' personalities. If you start moving around where the aperture of a barrel lens is relative to the front and back elements, I think you might seem some difference, but I haven't tried that beyond remounting lenses in shutters and finding that small thousandths of an inch difference make no difference.

Lots of people have used hats and caps for shutters.

I hope you're right that it won't make much of a difference. I assume it won't, or else this would be something that I imagine others would have discussed at greater length about these shutters previously.

But, just to play devil's advocate for a second, when you mount a shutter in front of the lens, you are changing the aperture location and size for fractions of each exposure. While the front shutter is opening and closing, it is acting as the aperture. And not only is the aperture location changed, in the brief fractions before the shutter is fully open, as it is expanding open, it is effectively a smaller aperture than the lens would typically have. I guess what I'm trying to ask is, do those moments during opening and closing impact the image at all? I hope not!

@TinCan, I will definitely test it once the shutter arrives, and I'll report results here.

To answer your questions, it's a 4x5 field camera, an unmarked 180mm f/3.5 petzval, and a packard shutter with a 3" opening.

[Tin Can]

no problem

[reddesert]

It does not matter. The main issue is to make sure that when fully open, the shutter does not vignette your lens.

First, the amount of time that the shutter is opening and closing is a small fraction of the total time.

Second, the shutter either in front of or behind the lens is not exactly the same, optically, as the aperture inside the lens. The aperture inside the lens forms the pupil or aperture stop that governs what rays can pass through the lens. The pupil is maximally out of focus. For optics reasons that are too complex to explain quickly, the shape of the pupil determines the diffraction pattern of the lens, which is why for ex a five bladed aperture will give you a different number of light spikes from a point source, or shapes of out of focus blur (bokeh), compared to a four bladed aperture. However, if you stick a pentagonal hole in front of or behind your lens (like using a vignetter in a matte box), you'll get a pentagonal vignette.

There is a lot of optics misinformation or half-information on the internet, and the idea that the shutter has a different effect depending on whether the lens has formed the image already might fall into that category. What matters is whether the shutter is vignetting the lens. Vignetting can change with front or back placement, depending on lens size and angle of view.

[AidanAvery]

@reddesert

Awesome. Thanks a lot for explaining that. That is great to hear, both because it reassures me that a front-mounted shutter will serve my purposes, like I had originally thought it would. And because it helps clear up the misinformation I had read last night.

I'm actually going to edit my original post with this response quoted at the bottom so that I don't perpetuate the same information that spooked me.

EDIT: I should mention that the shutter I purchased has an opening an inch larger in diameter than the front of my lens so that I avoid any vignetting problems.

[Dan Fromm]

One real risk -- vignetting -- was mentioned by reddesert in post #6 above. The lens receives and projects a cone of rays. A thin panel with a circular hole -- that's a Packard shutter -- in front of or behind the lens can block the outer part of the cone. That's vignetting. The farther the panel is from the front or rear of the lens, the more of the cone will be blocked.

In practice, OP, I think you're stuck. Packard or no shutter. Potentially lose some coverage. To find out how bad this might be, make a panel with the right size hole, set the camera with lens attached to a board up and look at what you can see on the ground glass. Then put your panel -- simulated Packard -- behind the lens board and look at the GG again.

[AidanAvery]

Hi Dan,

Maybe you didn't see my edit above. I got a packard shutter that opens to a diameter 1" larger than the lens itself. I will mount it directly on the face of the lens. It's not going to impede the lens's field of view at all.

[Drew Wiley]

Depends on the rigidity of the camera and its support, especially with respect to the front standard. The more weight there is, and the further forward, the more torque there is on the front standard, and hence more vibration risk.

Otherwise, I'll let those with actual Packard shutters spell out any other issues. You'll have to experiment, regardless.