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Old Nov 16, 2005, 3:59 AM   #1
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I'm thinking about getting a "big zoom" digital camera.( cannon 2is, panasonic FZ5 etc)

1. People keep telling me that "digital aint 35mm". To understand this you need to know about about how lens work. So i did a bit of research and I'm totaly confused.

Many so called "telephoto" lenses are actually retrofocal.

Many fixed lenses are actually telephoto lenses.

Many macro lenses aren't actually macro.

:?

The "big zooms" have a telephoto and a macro mode! What type of lenses/construction are these?

If anyone can shed some light on this it'd be much appreciated.

2. A more practical lens question. In the "big zooms" when you set the aperture when does it close. ie does the lcd gain up and show you the image with the aperture closed (or does the lcd just go dark) or is the aperture kept open until you take the picture (like film slr's do)?
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Old Nov 18, 2005, 1:12 AM   #2
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I could be wrong, but I am sensing some confusion with 'zoom' and 'telephoto'. A zoom lens simply has a range of focal lenghts, which may or may not be telephoto: an 11mm to 50mm is zoom, but not telephoto. 800mm is telephoto, but not zoom.

The original meaning of 'macro' was an ability to focus an image the same size on the film (or sensor) as the subject was in real life. (or within the same order of magnitude). Now, many people use it to simply mean a close focussing lens. Different sensor sizes, and the resulting images, tend to make comparisons difficult.

The construction of wide range zooms with macro focus capability is too complex for the likes of me. Look at the details of the number of lens elements and groups listed for one of these lenses, and try to figure out what they all do. Good luck with that.

Most, if not all the cameras you are talking about, keep the aperture open until it is time to take the picture. In some, the iris is also the shutter, so the sequence goes from full open, to closed, while the sensor is cleared, then open to the set aperture for the assigned shutter time. then closed for sensor reading, then back to full open.

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Old Nov 20, 2005, 8:08 PM   #3
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Thanks for your reply, Brian.

What you said about the aperture is dissapointing. Film SLR's work this way because they use mirrors and the film is more sensitive than eyes; but it would seem to me that a digital camera could compensate for the camera settings and different sensitivities etc (all stuff that the camera knows) and show the image that will actually be recorded (instead of making you guess).

As to the lens stuff, I did realize that not all zooms were telephoto, but, yes, I was equating all telephoto's as being zooms, doh. I think my problem is with the relationship between focal length and the minimum focal distance (and I was thinking of the non-telephoto zoom rangeas being macro? - not sure what I was thinking).

Correct me if this is wrong but I seem to recall that to get a lifesize image of the subject, the subject needs to be one focal length in front of the lens (or there abouts depending on the lens design).

The other thing I'm not sure about is "35mm equivalent"; in what ways is it eqivalent and in what ways isnt it equivalent? How will the digital result differ from film with the same setup ( equiv FL, f-stop, shutter, distance etc)? What setup would give the same results?

One glibly learns lots of rules for 35mm (like the "1 FL for lifesize") but I think some of these need to be re-interpreted and modified for digital (or, it just occured to me, that maybe f-stop is really 'equiv f-stop', shutter 'eqiv shutter' etc, so that the re-interpretations are already built in?)

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Old Nov 21, 2005, 2:20 AM   #4
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Some digicams are able to five you a WYSIWYG display. Mine does this by default in manual mode. The drawback is that, in poor lighting, or if you wish to have a dark looking photo, the display is so dark as to make framing difficult. I can optionally set the display to 'gain up' to show brighter, but of course, I lose the abiltiy to see the exposure correctly.

35mm equivalents are useful to get an idea of what a camera is capable of, and how a photo will look relative to what people are used to seeing. For a 2/3" sensor size, the actual focal lengths are about 1/4 the 35mm equivalents, so the min focal length on my camera is actual 7.2mm, which is equivalent to 28mm. f/stops are relative values to begin with, so are not adjusted. Just remember they are relative to the smaller focal lengths, so f/11, with a 7.2mm lens, gives you a very small hole for light to enter, hence the depth of field is greater. Shutter speeds are not adjusted. 1/500s is the same for film and digital.

Setting up a shot with digital to look similar to a film camera does require making some adjustments. Mostly, it is understanding the greater DOF of the digital. Again, using my camera as an example, at f/2.8, I have approximately 4 times the DOF as with my 35mm camera, so for a portrait, I would use about 4 times the focal length, so have to be that much farther away from subject. This can be a benefit if the subject is nervous about having camera close. Good for landscapes, too.

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Old Nov 22, 2005, 1:51 AM   #5
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"... the display is so dark ....." but is it any darker than the recorded image? For a nice bright scene do you see f/g b/g blurr and depth at the same brightness as the recorded image? or are you saying the displays lower resolution makes the scene appear darker than it really is?

I'm still processing the rest of your post.

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Old Nov 23, 2005, 1:54 AM   #6
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The display shows what the recorded image will look like, except that with the flash enabled, the EVF gain is increased to give a bright image for setting up the shot. (manual mode) EVF resolution is not really enough to tell what all the details are like, but will give enough to show DOF. Many say that optical VF in DSLRs is vastly superior, but my experience with 35mm SLR tells me it is too small an image to give much better than EVF. YMMV I very much like the info the EVF provides, such as live histogram, and being able to eyeball the exposure before taking the picture.

I guess old film habits die hard, as I don't go out and shoot several hundred pictures in a day (one exception to that) just to get a few keepers. I want my camera to allow me to set things up my way, and show me what the results are going to be like.

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Old Nov 29, 2005, 10:25 PM   #7
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DOF !!! That really sets the cat amongst the pigeons.
When it comes to digital there seem to be more views on this than cameras. I am sure I didn't understand half of it, but I've tried to put all the little bits together, and this is what Ive come up with:

This started off as ones of those "if you want to photograph a 1.2m tall child, this is what you get with a 100mm lens and this is what you get with a 50mm .." I kept the 50mm part and worked from there

For same composition, & perspective, & exposure.
(assumes same sensitivity to light for sensor and film)

Print Size 100*150100*150 mm
Viewing dist 450450 mm
CoC-print 0.13090.1309 mm
Print factor 4.16666722.22222 -
sensor24*36 5.334.5*6 mm
CoC-sensor 31.41593 5.33 5.890486 um
Focal length 50 5.33 9.375 mm
F#165.33 3 -
1/Shutter88 (5.33)^2 2503.111 1/sec
Ammt light 0.044389 0.044389 -
FOV width 2.121.91 m
FOV height 1.421.44 m
Object dist33 m
POI re lens1.51.5 m
Near point1.881.87 m
Hyperfocal dist 5.024.98 m
Far point 7.377.52 m
Depth of field 5.495.65 m
Infinity blur539.8 um
POI blur disk 539.8 um
infinity blur on print 0.2208330.217778 mm

Do these figures look reasonable? The ratios in the middle were only hinted at onsomesites, but seem to work.

I also noted these are at the opposite extremes for the two cameras ie: 35mm cant go any slower (realistically for subject) digital cant go any faster, or bigger aperture either. This one example suggests to me that in very few casescan digitalbe the same as 35mm film. I guess this is only interesting if your trying to reproduce an effect that is on an existing print, which is a long way from where this started
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Old Dec 5, 2005, 12:34 AM   #8
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Keep in mind that a telephoto lens does not "magnify" the image.

A telephoto lens simply picks out a narrower frame of reference.

For instance, a very long 600mm lens can pick out a bird sitting in a tree at several hundred yards, while ignoring everything else around the bird.

Whereas a 15mm wide angle picks up a huge wide angle vista, where you'd be lucky to pick out the bird in the tree because you'd be taking a picture of a hundred trees.

Anyways, most people don't realize what I just told you.

-- Terry
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Old Dec 5, 2005, 8:51 AM   #9
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[email protected] wrote:
Quote:
Keep in mind that a telephoto lens does not "magnify" the image.

A telephoto lens simply picks out a narrower frame of reference...
Huh!? In my mind, filling more of a frame with a given object, also increasing the number of pixels that make up a given object, is *by definition* *magnification*.
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Old Dec 5, 2005, 2:32 PM   #10
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Your right, if you think of magnification as "making something look larger".

However, a lens just focuses light.

A wide angle focuses a wide angle of light onto your camera sensor, whereas a telephoto focuses a narrow angle of light on your camera sensor.

- Terry
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