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01-13-2014, 03:03 PM   #1
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Understanding Lens Spec

Hello,


I'm just new in the field of photography and I just want to know if there is a way to correlate the lens spec (say 55-300mm) to magnification (say 3x, 4x etc.)? Thanks!

01-13-2014, 03:09 PM   #2
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The focal length corresponds to the field of view in degrees (not necessarily magnification). Check out this table for the conversions and look under the APS-C column:
https://www.pentaxforums.com/forums/54-pentax-lens-articles/93714-field-view-...d-645-6x7.html

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01-13-2014, 03:18 PM   #3
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QuoteOriginally posted by tadejesus2014 Quote
Hello,


I'm just new in the field of photography and I just want to know if there is a way to correlate the lens spec (say 55-300mm) to magnification (say 3x, 4x etc.)? Thanks!
When you say magnification, do you mean with respect to what compact cameras show? Or do you literally mean magnification as in, making small objects look big?
01-13-2014, 03:23 PM - 1 Like   #4
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You might be confusing a couple thing.
Little point and shoot cameras sometimes have things like "4x Zoom" written on them. This is merely the difference between the widest focal length and the narrowest that the camera offers, it is not "how close the camera can bring a subject". Zoom only means the lens allows many different fields of view. A 100mm tele lens brings far away objects "nearer" but its doesn't actually "magnify." This is why it is hard to compare P&S cameras to more advanced DSLR cameras - the P&S use terminology very loosely and in a confusing manner.

So lets move on to Magnification. The biggest magnification is actually achieved at the minimum focus distance (when focused as close to the lens as possible). The MFD in combination with focal length gives you a Magnification. A 1:1 magnification means that the object projected by the lens will be the same size as the actual object. The current Pentax macro lenses all allow true 1:1 macro. Many other lenses claim to be macro, but aren't. For example, the Zeiss macro lenses are only 1:2 (which is still acceptable for macro, but not as big a magnification as the Pentax DFA macro lenses), but some zoom lenses that have "macro" written on the box only give you 1:5. This is not truly macro, but it might be a smaller MFD than other zooms. You see, the MFD depends on the lens design, so not all lenses of a given focal length have the same MFD. This also means they don't give the same possible magnification.

What Adam linked you to is the Field of view table. Every lens has a given focal length. But it depends on the medium (film, sensor) how much it actually captures. Simply put, you put a classic wide angle lens on a very small sensor, and it will not be as "wide" anymore. For example, a 35mm lens on a Pentax Q will be a tele lens, not wide angle, because the smaller sensor does not capture the whole projected image. Each lens projects a circle on the sensor plane, but if the sensor is smaller, it will effectively clip the Field of View. This is why the Field of View is not the same as the Focal length. Focal length is a lens property, field of view is Focal length in combination with the medium (film, sensor) size. So if you know a focal length of the lens, you can multiply it by 1.5 to figure out its equivalent Field of View on a full frame/film camera. For example, 20mm lens on crop sensor will give you the same field of view as a 30mm lens on full frame/film. This is called the crop factor. Medium format cameras (like Pentax 645D) have a sensor bigger than full frame, so the factor goes in the other direction.

If you look at the Lens Database you can often find listed the focal length (in mm), the field of view (in degrees), and the maximum magnification (either as a ratio or multiplication factor). Here is the entry for the Pentax 55-300mm

And then one more thing to confuse you.. since Pentax at the moment is producing only crop sensor cameras, you can expect the magnification to "appear" bigger than it actually is. For example, the Pentax DFA 100mm can project a 1:1 magnification. On film, a subject might cover half the frame. But if you have the same magnification and same subject, due to the smaller crop sensor, the subject will cover more of the frame. This means its not truly "bigger magnification" but.. well, it appears as though it is.


Last edited by Na Horuk; 01-13-2014 at 03:36 PM.
01-13-2014, 04:19 PM   #5
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It's a simple question but photographers rarely think in terms of magnification (except in macro, as Na Horuk explains). The field of view data that Adam has pointed you to is the complete answer. But I grew up with a different background and I find it useful to have some concept of "magnification" at the back of my mind. If you want a quick "rule of thumb" to use, this is how I approach it: I grew up with 35 mm film cameras (24x36) and 50 mm lens on a 35 mm camera was generally accepted as "normal" view or, you could say, roughly 1x magnification. That being the case, a 200 mm lens would give 4x (200 divided by 50) and a 300 mm lens would give 6x (300 divided by 50). Switch to APS-C and you introduce a "crop factor" of 1.5 because the APS-C sensor is smaller (15.7x23.7 as opposed to 24x36 The crop factor is easy to calculate: 36 divided by 23.7 gives roughly 1.5). So now a 200 mm lens on an APS-C camera gives you a magnification of 200 divided by 50 multiplied by 1.5 which is 6x. And a 300 mm lens is roughly 9x etc etc. On APS-C, the 55-300 lens will give you roughly 1.6x to 9x magnification. Actually, once you know the relationship you will never do the rough calculation. You will "know" automatically what to expect from a lens.
01-13-2014, 06:16 PM   #6
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QuoteOriginally posted by Adam Quote
The focal length corresponds to the field of view in degrees (not necessarily magnification). Check out this table for the conversions and look under the APS-C column:
https://www.pentaxforums.com/forums/54-pentax-lens-articles/93714-field-view-...d-645-6x7.html
Thanks a lot, Adam.
01-13-2014, 07:09 PM   #7
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QuoteOriginally posted by tadejesus2014 Quote
Hello,


I'm just new in the field of photography and I just want to know if there is a way to correlate the lens spec (say 55-300mm) to magnification (say 3x, 4x etc.)? Thanks!

300mm is a very narrow field of view (FOV). You're unlikely to have ever had such a narrow field of view on point-n-shoot cameras.

Those 3x and 4x lenses usually 'started' with a 28mm equivalent, although sometimes they started with 24 or 35 (not exhaustive, but usually).

So... the long end of the 300 mm is 3x 'longer' than the longest of what you might be used to.


What are you actually try to take pictures of, and in what conditions? How far away, daylight, etc, etc?

Last edited by ElJamoquio; 01-13-2014 at 07:14 PM.
01-13-2014, 07:39 PM   #8
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QuoteOriginally posted by tadejesus2014 Quote
Hello,


I'm just new in the field of photography and I just want to know if there is a way to correlate the lens spec (say 55-300mm) to magnification (say 3x, 4x etc.)? Thanks!
Magnification is the ratio of image size to subject size.

Image sizes greater than the subject I.e. 1:1 or higher, are considered macro.

Image size = subject size x focal length / distance

Ratios of 2x 3x etc usually describe the ratio of a zoom lens between minimum and maximum focal length but not directly magnification

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