Photography Basics

[plettner]

What is the difference between adjusting the exposure on my camcorder and adjusting the iris? My cam only has manual exposure.
How do f-stops relate to all this? And does any of this have anything to do with focal length? I think I understand the terms but not how they are related to each other.
Please advise!!
Thanx

[Boh!]

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[This message has been edited by Boh! (edited 23 November 1999).]

[plettner]

Yeah, thanx. That's what I thought. Is the Iris on a camcorder a mechanical one like on an SLR or is it a different beast altoghter?
I have a TRV-99e which does everyhting (except make coffee)automatically. It has some manual overrides, ie. exposure, focus, BLC, etc. It is based roughly on the UK's TRV-65/66 (I think?) By knowing how the camcroder's system works, I would then know how to best use these manual overrides.

Thanx

[bcrabtree]

I reckon a trip to the public library is in order. You'll find a goodly number of books, by folk such as Jon (??) Hedgecoe, which are easy on the brain and will bring you up to a fair level on understanding reasonably quickly. Most, though, concentrate of still photography.

However, there are video books out there (if not so many), including one by CV contributor Steve Parker.

One (extended) point that I'll make here is about the inter-relationship between three things:

* aperture - the size of the hole through which the light passes into the camcorder),

* shutter speed - the length of time that the lens is open for (though this can be actually the amount of time for which the CCD imager is sensitive, rather than how long the shutter is open)

* Depth of focus - the distance over which the image is in focus.

First, if you decrease the size of the aperture or increase the shutter speed, less light comes in. And vice versa. Makes sense really, doesn't it?

Second, a smaller aperture (which is designated by a larger "f" number, say f16); produces a greater depth of field than a larger "f" number, say f4.

So, for instance, if you are trying to shoot a person and wish to have him/her in focus (but the background and foreground out of focus) you can do this by using a larger aperture (smaller "f" number). Conversely, if you want a large depth of field, with lots of stuff in focus, you'd go for a small aperture.

But, to get enough light in to record an image (or prevent so much light coming in that the image is overexposed), you have to change the aperture setting in response to any change that you make to shutter speed, and vice-versa.

With a camcorder in fully automatic mode, the camcorder decides on both settings. In program mode, you (perhaps unwittingly) choose either to give priority to shutter speed (perhaps choosing a fast speed to catch action) or aperture (possibly narrow for extra depth of field).

In fully manual, you pick both and, the camcorder will have a way of warning you if the combination you choose is going to cause the recording to be under or over exposed.

Oh, and one other thing to know. A wide angle lens has a greater depth of field than a telephoto lens.

So, if you want absolute maximum depth of field, you'd shoot at the wide angle (zoomed out) end of the camcorder's zoom lens.

But, if you want to, say, isolate a person or object (with foreground and background out of focus), you'd shoot towards the telephoto (zoomed in) end of the lens, with a wide aperture.

Oh, one other little thing - a fast shutter speed (well, in an SLR type wet-film camera, anyway) is best for shooting with longer lenses (particularly if you are hand holding) because it cuts out camera shake from your unsteady hands. Some camcorders, though, have first class auto systems for overcoming camera shake.

Now get ye off to the library for a good book or three.

And, please come back here in a fortnight's time to tell us what you've learned, and to recommend some books that others should read.

Bob C

[This message has been edited by bcrabtree (edited 05 November 1999).]

[gadget]

Plettner,

The way lenses and exposure works in video and is broadly similar to stills photography. This is because video can be thought of as the rapid display of successive still images.

The focal lenght of a lens is the distance from the pole of a lens to a plane, where a sharp image of an object can be seen when the lens is set to infinity - In other words, the distance from the centre (usually) of the lens element to your CCD (chip) when the lens is focused at infinity. The focal length determines the relative scale of objects seen in the image (size & perspective) and the angle of the field of view. It does not directly relate to the physical length of the lens or exposure.

Exposure relates to the intensity of light (on your CCD) over time. This is affected by the iris and the (electronic) shutter.

The shutter in a video camera is a measure of how long the CCD is 'active' during each seuccessive field scan. For PAL there are 25 frames (50 fields) to a second, so the shutter is never set slower than 1/50th
of a second. Just as with stills photography, a high shutter speed will 'freeze' rapid movement more - (this is often not desirable when filming as it can cause 'jerky' movement). I think some consumer camcorders have a set (1/50th) shutter speed(?).

In lenses with adjustable irises, the 'f-stop' is a ratio, (focal length of the lens)/(diameter of iris (or aperture)). This means that the higher the f-stop number, the smaller the aperture and vice versa. A large diameter iris = more light through lens = smaller f-stop no. A lenses 'speed' or f-number is the focal length/maximum diameter of the iris.

Exposure can also be affected by 'gain' (expressed in db's). This is an artificial amplification of the video level and will therefore introduce noise to the image. It should only be used when absolutely necessary.

Hope this helps, I would suggest a good photography book for further investigation.

Regards,

Ben (Gadget)
BITS Ltd. (www.blackice.co.uk)

[gadget]

Apologies for repeating the same info! Bob's reply must have passed me in cyberspace.

Regards,

Ben

[Keith Head]

Did I read correctly, that the shutter speed can not be set slower than 1/50th second?

Not so on my Canon MV1; In TV mode you can select shutter speeds of 1/6th, 1/12th or 1/25th in addition to the fast speeds.

Incidentally, a major little-promoted virtue of DV is its automatic recording of exposure data for each frame. For any frame you can later call up the shutter speed, the F stop and the amount of exposure gain used. My only regret is that you can't check all of these settings before making the exposure; on the MV1 you can know the F stop or the shutter speed beforehand, but not both.

Keith Head

[Alan Roberts at work]

Does the MV1 genuinely integrate light for 1/6, 1/12th, 1/25 second in these modes? Or are you just recording every 8th, 4th, 2nd frame? You can tell by checking the exposure level, if 1/6th really means integration for the full 1/6th second, you should be able to close the iris by 3 stops for the same exposure as 1/50th, and you'll get very blurred motion. This is the trick normally used on camcorders to exaggerate the sensitivity specification.

[Keith Head]

The MV1 seems pretty genuine to me. The exposures at 1/6, 1/12 and 1/25 all appear to me to give the correct light increase relative to 1/50 and it is apparent in the viewfinder before recording. They are very useful for low light levels and static situations. They can also be useful when copying from projected film.

If there is movement in the scene the blurring can be a nuisance, but if you like the effect it's right there at your fingertips.

Keith Head

[plettner]

Still trying to get to the library. I live a little out of the way from one. Yes - I will get to it eventually. I have some books on order from Amazon.com also. I'll read them (when they arrive) and let everyone know how I go...

Thanx Everyone!