Math and Lens Aperture Opening Size

[_Paz_]

Take a 400mm prime lens with a maximum aperture of 5.6, a minimum aperture of 32, and a filter size of 77mm.

This lens will not communicate with the camera if the lens is set anywhere but 5/6... but the sweet spot is f/8, perhaps f/10 in some instances.

Is there a way to figure out the size the opening of the aperture would be at f/8? And f/10?

Then, if you took a piece of flat black cardboard or construction paper and cut the right size hole in it, would it be better for the stop down paper to be between the lens and the camera body or at the filter end?


What shape would make the best bokeh? Round, hexagonal or octagonal??? I've seen some sweet heart shaped bokeh created by bokeh filters but I can see how a little too sweet could go saccharine pretty quickly.

Paz

[momoffduty]

Every lens is different for the sweet spot. Look at online reviews of the lens to find the best f stop. Generally it is a stop or two above. I have the 50mm 1.4 and like to shoot that at 2.8 or 3.5 depending on DOF I need. If it is people based then you have to account for the rows of people to get everyone on the same focal plane.

I've made the cardboard cut outs and have the star and heart. A novelty? Yes. My Branson video of fireworks I used the cut outs.

[sidd finch]

I've made the cardboard cut outs and have the star and heart. A novelty? Yes. My Branson video of fireworks I used the cut outs.

That reminded me of an episode of Poirot I watched a while back. They used the old technique of painted glass and flame to make the actors look like they were in the desert. As the camera pans back you can see that some of the scene was on painted glass. This is the same effect they used to make Gone with the Wind.

The effect is at about the 1:10 to 1:50 mark.

[youtube]http://www.youtube.com/watch?v=zC7lVOOHEgQ[/youtube]

Sidd

[_Paz_]

I found this formula online:

a 100mm focal length f/4 lens has an entrance pupil diameter of 25mm

a 200mm focal length lens at f/4 is 50mm


=================

Therefore, does it follow that a 400mm f/4 should be 100mm?

That's bigger than the end of the lens, which holds a 77mm filter...

But, as the camera's max f/stop is 5.6, that might be right.

If a 400mm f/4 should be 100, then would an f/8 on the same camera be 50mm?

If this is the 'entrance' pupil, should an opening this size be next to the camera body

or is it OK if it is out at the outer end of the lens?

[_Paz_]

Mom,

Reviews indicate the f/8 is super sharp and f/10 is"AMAZING". But I'm stuck at f/5.6 No way to change it in camera. It causes an error message and won't take the shot.

I've read I can 'fix' this by cutting out the right size opening and taping it on the front of the lens. But what size and shape hole?

Stars, butterflies, hearts - all fun for parties!!! But maybe not best in the out of focus areas behind Isaac the Newt, or a squirrel or dove or heron.... I need the best 'every day wear' shape.

I have another 12 days to decide if I want to keep the lens. I shot some video today at f/5.6 but I think it could be sharper and I'd be happier.


Sidd,

Interesting!

[momoffduty]

Patrice, sorry you lost me on the calculations. I am not sure on the question of the opening. What are you shooting with a 400mm? Seems a tad long for video. And why won't your camera allow the aperture change?

For the bokeh cut outs, this is the tut I followed:

http://www.howtogeek.com/101728/the-how ... oto-bokeh/

[Bob]

Yes, what camera and what error message. Inquiring minds want to know.

The f number is defined to be the focal length divided by the entrance pupil diameter. To calculate the entrance pupil diameter, divide the focal length by the f number. For example: 400mm / 5.6 equals 71.4mm.

The entrance pupil doesn't necessarily equal the diameter of the lens opening. It can differ from the actual physical opening due to the refractive properties of the lens. Also, the entrance pupil is generally located inside the barrel of the lens, not at the front element surface where you attach filters. The precise location will differ for each lens and may change as the focal length of a zoom lens changes. The position is generally further back with wide angle lenses and closer to the front with telephotos.

You'll want to place your homemade aperture stop on the front of your lens, never the back.

Do a test shot to confirm that everything is good. You may see vignetting. One thing to be aware of, depending on your camera's focusing mechanism, you may not be able to use autofocus with the cardboard aperture stop on the lens. Check your camera documentation for f stop limitations. You may need to use manual focus or focus without the aperture stop and lock the focus before placing the stop on.

[Ron Hunter]

Patrice, the formula that relates aperture and focal length is:

"f number" = focal length of lens (mm) / lens aperture opening (mm)

For example, a 400mm lens with 200mm lens aperture opening produces an f number of 2, or f/2. Since the (mm) cancel, "f number" is dimension-less. With a large f number, like 22, you can see that the lens opening must be really small for that division to yield 22. Conversely, a small f number like 2.8 means the aperture opening must be big to cause the division to only yield 2.8.

If we rearrange the formula we can solve for aperture opening size:

Lens aperture opening (mm) = focal length of lens (mm) / "f-number".

For example, I have a 12-35mm lens that is "f/2.8". At 12mm and f/2.8, the lens aperture is (12/2.8) = 4.3mm wide. At 35mm and f/2.8, the lens aperture is (35/2.8) = 12.5mm wide. Same "f number" but because the focal length increased, the lens aperture must also increase.

[_Paz_]

What are you shooting with a 400mm? Seems a tad long for video. And why won't your camera allow the aperture change?

Birds, Mom. And any other wildlife that won't let me in their comfort zone. In my own yard I've seen deer, red fox, grey fox, rabbits, quail, doves, chipmunks, squirrels, groundhogs, tortoise laying eggs, woodpeckers, green heron, great blue heron, all sorts of small birds, rattlers, copperheads, moccasins, blue tailed lizards, giant spiders, butterflies... over a couple of decades, mind you... I looked up the Audubon Society in my state to see where I might go to find birds... and one of their field trips includes the country road I live on.

Yes, what camera and what error message. Inquiring minds want to know.

You're funny, Bob!!!


Canon 7D and the Canon mount version of the lens the guy used who shot the hummers I posted here under "Humdinger!".
That's a Sigma 400mm Macro APO HSM (Hyper Sonic Motor). No zoom. Lens length does not change when focusing.

http://muvipix.com/phpBB3/viewtopic.php?f=1&t=14155

The lens gets rave reviews online for image quality, with this one saying it outperformed a Canon L:

http://www.photozone.de/reviews/306-sigma-af-400mm-f56-hsm-apo-macro-test-report--review

I'd already bought it (can return it within time limit) on Ebay, from Japan, before I came across this on the web:

there is an issue as regards using these lenses on Canon DSLRs. Canon altered the lens camera interface with their DSLRs, and whilst it did not effect the Canon EOS lenses, some lenses with back-egineered lens mounts such as the Sigma 400mm and 300mm Apos wouldn't operate with them. Whilst Sigma did re-chip quite a lot of the lenses that owners had problems with, there is still a lot of lenses, perhaps the majority sold secondhand, that were not re-chipped. So a lot of them won't work with most Canon DSLRs. My understanding is that some just stick to their wide open aperture and won't stop down any further, but some create an error99 malfunction with the camera. It maybe possible that the last model Sigma 400 5.6 HSM APO Macro models didn't have this issue and I believe that these were the best performing lenses. So be cautious if you see a used one of these lenses for sale and check that it will work with your camera."

The error message I've gotten is "01". No 99, but I did read up on whether that was a camera killer - no - and how to get the camera working again before I tried it. And I tried the lens in manual first, small steps, all seems OK except I can't change the aperture.



Ron and Bob,

Thanks for the math lessons! Looks to me like 50mm should be f/8 and 40mm opening should be f/10. I've found a sheet of black construction paper. I'll get out my nearest size step up rings to make the patterns for the cutouts - and put them at the outer end! I guess I'll just cut a circle.

[Bob]

That's too bad about the 01 error. That's a result of Canon changing the codes which control the lens operation. That's the problem with using lenses not made by the camera body manufacturer. You aren't guaranteed future compatibility with any changes the camera manufacturer may make. Newer Sigma lenses use programmable memory and might be able to be updated for compatibility by Sigma, but older model lenses, like the lens you have, used non-programmable Read Only Memory and Sigma does not rechip those lenses.

The external stop should help. Hopefully, you'll still be able to autofocus with it on. Let us know how your tests turn out.

[_Paz_]

From what I've read, Sigma re-chipped by request until they ran out of chips.

Martin Melchior found some info on how to DIY re-chipping it - translated it from German and assembled the info on this page:

http://www.martinmelchior.be/2013/04/conversion-of-old-sigma-lens-to-work.html

Apparently Version 2 is the one to use. That's pretty much all I was able to understand! I can operate a screw driver, even a teeny - tiny one, and I can solder a copper pipe but that's about as far as it goes!

While trying to use step up rings as a pattern it hit me - why not use step up rings??? So I made some comparison video, then selected best focus shots yesterday, but have been busy with other things today. I let 52mm opening represent f/8, 37mm = f/10, and 25mm = f/16.

None of them cause vignetting. Only the 52mm opening will allow auto focus. Even using a (video - heavy) tripod, touching the shutter button causes so much movement I haven't gotten a good still shot yet.

Using the Aputure V Controller in Stills mode I can change Exposure Compensation, move a 'box' around the screen to choose point of focus, then digitally zoom in 5 or 10 times to help figure out focus, and take one still shot (I think it should do more).

Using the V Controller in Video mode I can start and stop video and when NOT filming, I can digitally zoom in the 5 or 10 times. You can also select up to four objects and have the focus move from one to the other. Useful if you are filming a conversation between some people at a table, I suppose, but I haven't personally used it yet.

Using the big wheel on the unit moves the internal focus mechanism one click at the time in either stills or video mode. You can adjust the wheel while videoing - but not in the zoomed in mode. The unfortunate thing is, this lens seems to have the most shallow depth of field of any lens I've ever used. Maybe it is because it is so long. 400mm is far beyond anything else I have.

As I haven't had success in taking stills, I can't say I've seen "L" quality. I'm going to order batteries for an old Canon RC-1 remote control gizmo I've had for decades. Hope it still works!

I did get some video of moving geese that were a minimum of 100 feet from me - possibly 200 feet, if the scale of miles on a local map is correct. It was the first time I took it out and on my Big Foot monopod. Too shaky. I need to go back with the heavier video tripod.

Here is some video I took in my front yard with a series of step up rings that went down from 77 to a 25mm opening - f/16, if my math is right. I do think this is nice... but I wish I'd found a bee or a spider or even a fly's eye to check out. Something really good for a 400mm macro test. The len's minimum focal distance is 63 inches. This was from about six feet away under heavy grey clouds.

I think I'm going to keep the lens. It does everything the seller said. I think the condition is even better. Original box, visually untouched case, mint apparent condition. I do wish I could change aperture - especially for the macro shots, but I paid under $300. That Canon 100 - 400 I've wanted so long is $2,200. I'll put up with this for now and if I can ever upgrade to that one, then I'll sell this one.

[_Paz_]

Still testing. Vimeo says it doesn't understand my color. My Canon 7D is probably set for RAW. Should I change it to sRGB for video?

Also, with the 400mm lens I have to leave the camera's aperture setting at f/5.6. I've been using one of the 'safe' modes, Aperture Mode.

When I change to a smaller opening, such as f/10 (more or less) I've noticed the video gets very grainy. Even though I have the ISO set at 100, is it possible the camera is changing the ISO when videoing?

thanks,

Paz

[Bob]

The 7d records video using H.264 in a .mov container. Video shouldn't care whether you are set to RAW or something else. The 7D does something a little odd in that it records color with the SD matrix. But, it's all flagged in the mov file so it should decode properly. Are you uploading your files to Vimeo as they came from the camera or are they edited and re-encoded?

The camera shouldn't be increasing the ISO if you aren't using auto ISO. Try setting Highlight Tone Priority off and see if that makes a difference. And, try ISO 160.