A question of optics

[AjohnwParticipant @ajohnw51620]

The sensor size aspect isn't too much of a problem to work out but it's based on vidicon tubes, probably 'cause it makes them sound bigger.

1/2.83 is the inch size of an equivalent vidicon tube so that would be 0.3534 diameter. The actual diagonal size of the sensor will be close to 2/3 of that. So 0.2356 – 5.98mm. May as well say 6mm. It's best to stick with diagonal for this as aspect ratios vary. The 2/3 part does usually work out.

So if you want say 50mm to fill the sensor diagonal you need a mag of 6/50 = 0.12.

So the next thing is to work out the mag of the lens on the camera. That can be done by photographing a rule on the diagonal at various focal lengths at some working distance. It's not unusual for the magnification to be less than expected as the focal length gets longer. Moving from say 20mm focal length to 40 should double the magnification.

I backed out and just bought some cheap close up lenses to try them. You could set your camera up at some working distance that the long end can manage. Not as close as it can be to leave some scope for focusing. Shoot the rule on the diagonal at various focal lengths and calculate the actual magnification. Then use those numbers to pick a lens to stick on the end or use them in the little calculator I posted a link to earlier to see what effect various close up lenses will have. Ideally you need to be able to shoot the entire clock or zoom in to small parts of it without taking the close part off. Might be impossible might not. Distance may be a problem at wide settings.

Not sure about the effect on working distance when close up gear is fitted.

If the lens on the camera moves when it's focusing it's a bad idea to stick another heavy lens on it. The other problem with that approach is the distances the back end of a lens are computed to focus over – a fixed distance of circa 50mm. They will work at others but at some point it wont be too good.

Michael is assuming that the entire lens is moving when focusing – it might be or it might not be.

It is possible to not use both lenses focused at infinity but there will be limitations due to the focal distance the camera lens can cope with. It has to refocus what ever is coming out of the other lens. That applies in the case Michael mentions when no internal parts of the lens attached move differently to others. In that case the focal length setting on it wont matter.

Arggg no time to correct any weird typo's I may have made.

John

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Edited By Ajohnw on 05/08/2016 11:40:36

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ajohnw on 05/08/2016 11:31:32:

Michael is assuming that the entire lens is moving when focusing – it might be or it might not be.

.

I checked the specification of Sam's 100mm Macro lens and the answer is …

It has Internal focussing / Floating elements

Therefore my comment about the helicoid is irrelevant in this case.

MichaelG.

[Sam StonesParticipant @samstones42903]

Thanks for all your input John.

Since there was a (2^nd page) thread glitch, I’ve only just found your latest and very absorbing post. Some of this is leaving me behind and I’m tending to lose sight of what I’d like – i.e. some good close up video without spending too much.

My earlier attempts to get close to the skeleton clock’s escapement was through the Sony 405 Handycam. It boasts a 60x magnification although that turns out to be a combination of optical (30x) and the rest digital zoom. See this single frame (Snapshot) image from the Handycam.

I feel sure that the Legria can be adapted to take better shots.

I’m pleased to note that (as you remark), it’s not very wise to hang a heavy lens onto the front of another, especially if the front end moves when focussing.

As it happens, the exposed section of the lens of the Legria HF G40 is quite rigid, and would probably support the weight in a nose-nose (lens reversal) arrangement. I still think it’s a good point to make for us with less knowledge.

Since I don’t (yet) have a 58mm male-male reversing ring, none of my test shots were taken with the lens fixed to the camera.

It doesn’t look as if I’m finished yet.

Thanks again,

Sam

Edited By Sam Stones on 06/08/2016 22:44:09

[AjohnwParticipant @ajohnw51620]

I just had a look on the web to see if close up lens calculations are shown clearly anywhere. The wiki looks to have made a good job of it.

**LINK**

No escaping magnification though. That aspect is pretty simple. The sensor in your camera has a diagonal of 6mm. If you want to shoot an object 50mm across on the diagonal the mag needed will be 6/50.

Notice the comment about what happens to the max focus distance – same as the focal length of the close up lens. I feel like a bit of an idiot for not pointing that out.

So Renox's 2.5 dioptre has a max focus distance of 400mm. With a focal length of 75mm at that distance the mag will be 0.075 x 2.5 = 0.1875 so the max object size on the sensor diagonal would be 6/0.1875 = 32mm

With the lens at 6mm the max distance would still be the same.. The magnification would be 0.006×2.5 = 0.015 so 6/0.015 = 400mm would fit across the diagonal. I picked 6mm as that is the equivalent of a standard lens on the camera.

In practice you might find it works well over a range of something like 10 to 50mm lens focal length settings.. That is a guess though and it might still work well at longer lengths. Short lengths are probably limited by wide angle effects beginning to creep in.

The wiki also gives sums for shorter distances. Some people reckon that plain glass close up lenses are ok with dioptre of 1 and that they get dubious above that. Decent ones will have curves on both sides of the glass.

Not on commission but the nice thing about the Raynox is it's quick and easy to take on and off as it clips on. I think that the other one they do is 4.5 dioptre. The figures are on their web site.

I use a cranky old calculator that even has an X<>Y key and sometimes it acts up so do check the sums. I just don't like the newer ones. Too much button pressing.

John

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Edited By Ajohnw on 07/08/2016 13:41:24

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