25mm f1.4 CCTV lens

M

matt9b

Well-known member
Hi everyone,

I bought this off ebay:
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=170607739110&ssPageName=STRK:MEWNX:IT

but it only seems to be able to focus on objects about 1cm away from the lens!

Even if I turn the focus ring the focus seems to remain at 1cm. I can't get it to infinity or anywhere near infinity.

The listing states 'minimum object distance: 30cm'. So I would guess that my lens must be a duff.

Has anyone else bought something like this and can comment whether this is normal and I need some sort of diopter, or whether I have bought a duff lens?

Granted, its for a GH1 and I'm using a 7D, will this make much difference? The sensor size is similar on both cameras. I've also tried varying distance to the sensor to no avail. Can this make such a radical difference?

Thanks
 
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It won't really work on a 7D. You will get an image circle in the middle of a black ring because the lens wasn't made for sensors that large....
I don't know about focusing, but I would bet it has to do with the lens being too far from the sensor. I wouldn't say it is work using though....
 
Yeah, no possible way a c-mount lens is going to work on a 7D. It would need to be mounted over half an inch inside the camera body to have a prayer of focusing properly, and even then the image circle it makes won't be big enough to cover the 7D's sensor so you'll have big black vignetting all around.
 
JPM Photography said:
I wouldn't say it is work using though....
Click to expand...

That's what people thought about the old iscorama anamorphic lenses before someone realised they could be used on new DSLRs for video :happy:

Cheers guys, you're right about the focusing, however, I'm not sure the rest is 100% true. I'm sorry for contradicting you, I've just done lots of research :laugh:

Just in case this helps anyone:

In fact, there are some c-mount lenses that have been used on 7D without any problem. For example, cine 16mm lenses are large enough for the sensor. Why else would they make a 7D -> C mount lens adapter? :) I hear what you're saying but the fact that this lens is able to focus with no black ring shows that it is not the problem.

Thanks Barry for the focus explanation :D I had come to this conclusion through practical experiment. Nice to know my experiments were right! The reason this particular lens doesn't focus further than a few cm, is because of the flange distance... i.e. the 7D's sensor is just too far back in the camera.

This also explains why Panasonic / GH1 / GH2 users have such a large choice in lenses. The GH1 sensor is right up close to the front of the camera body.

However, I'm not convinced there will be a problem with black circles on all c-mount lenses.

Feel free to contradict me if you think I'm wrong :laugh:
 
What cine 16mm lens is large enough to cover the 7D sensor? I can't find any that are worth using (shorter than about 30mm) that will even cover the GH2's sensor. I imagine if you're talking about 50mm or above they might cover, but even my Super16 zoom won't cover the GH2's sensor.
 
Because he has the lens so far away from the sensor the imaging circle (cone really) is larger, thus probably able to cover the sensor...just not focusable any more than macro-ish.

Try this experiment, take your lens off the camera, out of the mount.

Turn live view on so your sensor is exposed. If you feel daring put the back of the lens inside the throat of the eos mount (don't scratch your sensor). Set your lens to midway through the focus range (15'?) move the lens until you get something 15' away in focus. That's how far in the lens would need to sit. I know you will get a lot of hazing if there is a lot of light in the room...

Your imaging circle might not cover now though...
 
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