Lens Crop Factor

POTN has taught me that the crop factor for my T1i is 1.6. Thus, a 50mm prime is actually shooting at 80mm and an 85mm prime is approximately 135-136mm.

Does this same rule apply for a telephoto lens? Does my kit lens actually shoot at 29-88mm?

POTN is a wealth of information and a priceless resource. I am learning so much here. I searched first, but did not find an answer to the above, so hopefully this is not a duplicate/redundant post.

Thanks.

Regards,
fiVe

On the T1i the angle of view of your lenses is their current focal length times 1.6 all the time, every time.

So your kit ZOOM lens (not telephoto), does indeed have the 35mm equivalent angle of view of a 29-88mm lens. It is still an 18-55mm lens though.

Similarly the 35mm equivalent angle of view of your 55-250 lens is 88-400mm.

This is why crop cameras are usually preferred for birding and other super-long telephoto lens activities, they make it easier to focus on stuff further away and make those things look bigger in the final photo, as if the subject was 62.5% closer.

Telephoto btw, just refers to lenses with a focal length longer than ~60mm. There are three basic focal length groups: 40mm and below = Wide Angle, 45-55 = Normal, 60 or more = Telephoto
(The missing 10mm are the ones nobody really agrees on). These groups are also broken down further by many people, into Ultra-Wide Angle, Wide Angle, Normal, Short Telephoto, Medium Telephoto, Long Telephoto, and Super-long Telephoto. The exact ranges of these groups are debatable.

Zoom refers to a lens that can change focal length.

arentol wrote in post #11896443
On the T1i the angle of view of your lenses is their current focal length times 1.6 all the time, every time.

So your kit ZOOM lens (not telephoto), does indeed have the 35mm equivalent angle of view of a 29-88mm lens. It is still an 18-55mm lens though.

Similarly the 35mm equivalent angle of view of your 55-250 lens is 88-400mm.

.

^^^

Key specific wording to properly describe what 'crop factor' does, and does not do, highlighted in red and especially key wording also underlined.

Journalists and even camera companies often do not help alleviate the misunderstanding, but instead perpetuate it!

why are you worried about crop factor?...i think most of us could go thru life without ever having to apply the 1.6X to a lenses focal length...unless you're coming from a film background...or moving to a FF camera...forget it until it matters

The best thing to remember about crop factor is this - the only reason it exists is because when DSLRs came out, most users were coming from 35mm film and needed a point of reference to compare what they were now seeing on their APS-C camera to what they were used to seeing from their 35mm film camera.

"So using my 50mm lens on my D30 digital camera has the same field-of-view as an 80mm lens on my EOS 650 film camera? Got it."

If instead of 35mm film, people standardized on using APS-C sized film over the last few decades, then we all would have grown up learning that 20mm was "wide", 28mm was "normal", 50mm was "tele", etc., and we wouldn't have had to come up with a crop factor to compare our DSLRs to film cameras.

Your 50mm lens is a 50mm lens, no matter if it's on a T1i, a 5DmkII, or a ham sandwich. The only thing that changes is it's field of view. If you came from a 35mm film background, you'd find it tighter framing on your T1i. If you're not from a 35mm film background, then I would just forget about it. You'll learn through experience which focal lengths get you the results you want on your camera.

Cheers!

arentol wrote in post #11896443
This is why crop cameras are usually preferred for birding and other super-long telephoto lens activities, they make it easier to focus on stuff further away and make those things look bigger in the final photo, as if the subject was only 62.5% as far on a 35mm frame.

Corrected.

If you've never shot with a 35mm film camera (or full frame digitial) and learned to relate the focal lengths to that, don't worry about it. The only reason to make those conversions is for people who are used to "seeing" a particular focal length behave a certain way on their old cameras, but now are changing to using "crop sensor" cameras and need to adjust their lens kit a little.

All you really need to know is that your 18-55 is a slightly wide angle to short tele zoom, your 50mm is a nice short tele portrait lens, and your 55-250 is a short tele to long tele zoom.

You can see all this through right your viewfinder. If you ever find that 18mm isn't wide enough, look for something offering a shorter focal length (zooms starting around 12mm, 11mm, 10mm, even 8mm now). If you ever find you want more reach than you're getting with 250mm, look for a longer tele, either a prime such as a 300mm or 400mm, or a zoom that goes to 300mm, 400mm or 500mm.

A 50mm lens is always a 50mm lens. What's different is how it performs (angle of view) on different formats. If you put a 50mm on a point n shoot digital with a really tiny sensor, it can be a powerful tele lens. On 1.6X crop sensor like your camera (image area about 15x22mm), it's a short tele. On old 35mm film and current so-called full frame DSLRs (image area 24x36mm), it's a "standard" or "normal" lens that's not wide or tele at all, sort of what the human eye sees.

If you were to mount a 50mm lens on a medium format film camera with a large image area (45x60mm, 60x60mm, 60x70mm and others) it will behave as a wide angle lens. Put it onto a large format camera (4x5inch, 5x7inch) and it will act as an ultrawide lens.

Yet it's always still a 50mm lens. It needs to be designed differently for different formats, to produce a large enough image circle to completely cover the film or sensor, whatever size that might be.

But someone who is new to using a DSLR really doesn't have any preconceived notions of how a 50mm lens might have worked on other image formats, so to them it's just a nice, short tele, portrait lens.

kaptnkain wrote in post #11896553
Corrected.

Yeah thanks for that. Stated it totally backwards. It felt wrong as I typed it, but I am so tired I couldn't figure out why at the time.

Zoom. Not telephoto. Got it.

Wow! Great info and fast, too! Thanks much!

fiVe wrote in post #11896366
POTN has taught me that the crop factor for my T1i is 1.6. Thus, a 50mm prime is actually shooting at 80mm and an 85mm prime is approximately 135-136mm.

Does this same rule apply for a telephoto lens? Does my kit lens actually shoot at 29-88mm?

Somehow you have learned "crop factor" myths that are totally incorrect.

Please read the following article very carefully (and several times if need be). I think you will be able to answer your own questions afterwards.


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The "crop factor" is a reference number that relates to the difference in film or sensor size (known as the camera's "format") between two cameras like the Canon 7D and a 35mm film (or a so-called "full-frame" digital) camera. Let me list the facts:

35mm film cameras and so-called "full frame" DSLRs have a film frame or sensor size of approximately 24mm X 36mm, while the Canon 7D has an APS-C sized sensor, measuring approximately 14.9mm X 22.3mm. The other Canon APS-C format cameras - starting with the D30 in the year 2000 and progressing through all of the "digital Rebel" xxxD series, the xxD series, and today's 7D - all have sensors that are sized similar to that in the 7D.

When camera manufacturers started designing digital SLRs (DSLRs), they decided that the DSLR bodies should be about the same physical size and configuration as their 35mm film SLRs. For that reason, they concluded that they could use the line of lenses they already had for their 35mm SLRs on the new DSLRs.

All lenses designed for 35mm film cameras project an image circle onto the film that covers a 24mm X 36mm rectangle. The 35mm camera records the portion of that image circle that is defined by the opening behind the shutter for the film (24mm X 36mm in size). A digital SLR with an APS-C sized sensor only records the smaller area (approximately 14.9mm X 22.3mm) of the image circle projected by the same lens.

When you put a 100mm lens on a 35mm film camera and make a photograph, then put the same lens on a DSLR such as the Canon 7D and make a similar photograph - same subject, same position for the camera, and same focal length - and then enlarge both photographs to the same size print (4 X 6 inches, for example), it will appear as though the photo from the Canon 7D was taken with a longer lens. That is because the image recorded by the Canon 7D was of a SMALLER PORTION of the image circle projected by the lens - cropped, if you will - compared to the image recorded by the 35mm camera.

The special lenses made by Canon for the 7D (and other Canon APS-C cameras starting with the 300D - the first Digital Rebel) are called the EF-S series. These project a smaller image circle, making the lenses less expensive to design and produce in wide-angle and extreme wide-angle formats. The EF-S lenses also project deeper into the camera than the EF specification allows (the "S" referring to "Short back focus), allowing for less expensive wide-angle lens designs. However, an EF-S lens set to 40mm will produce the exact same image as an EF lens set to 40mm if both lenses are used on the same APS-C format body and both lenses are focused at "infinity". Focal length is focal length, period.

Now to the primary point that I want to make: NOTHING about lens EVER CHANGES when you put it on different format cameras. Focal length never changes. Aperture range never changes. The only thing that would change is the apparent field of view, and that change is not a function of the lens but it is a function of the size of the sensor or film that will record the image.

The "crop factor" calculation for "35mm equivalent focal length" has only one valid use. That is for comparing the field of view of lenses used on two different format cameras.

Here's one common example: Joe took a photo of Mount Rushmore with a 35mm camera from a particular place using a 200mm lens. You want to replicate that photo with your Canon 7D. What focal length do you need to do that from the same location that he took his photo? Divide the 200mm by 1.6 and you get the answer - 125mm.

Here's another popular example: Mary Sue has been using a Canon SX120 IS point-n-shoot camera and is wanting to use a Canon 50D DSLR. She is, of course, interested in what focal lengths she would need to keep the versatility of the SX120 camera's 10X super-zoom lens. The SX120 lens is actually a 6.0mm to 60.0mm lens, but the advertising also shows the "35mm equivalent" focal length range as 36mm to 360mm. To know the focal lengths needed for the 50D, merely divide the "35mm equivalent" values by 1.6. In other words, Mary Sue would need 22.5mm on the short end and 225mm on the long end for the 50D to have the same field (angle) of view coverage as her SX120 IS camera.

The "crop factor" is NOTHING MORE than a REFERENCE between the two camera formats that lets you compare the field of view of particular focal lengths between the two formats.

The "crop factor" (as related to using lenses essentially designed for 35mm SLR cameras) is always given assuming that the 35mm format (24mm X 36mm) is the reference master. Something to realize, though, is that the 35mm film format is not, never has been, and never will be the "master" format against which all other camera formats are referenced. It is simply the format of the cameras that have also evolved into today's commonly used digital SLRs.

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arentol wrote in post #11896443
Telephoto btw, just refers to lenses with a focal length longer than ~60mm. There are three basic focal length groups: 40mm and below = Wide Angle, 45-55 = Normal, 60 or more = Telephoto
(The missing 10mm are the ones nobody really agrees on). These groups are also broken down further by many people, into Ultra-Wide Angle, Wide Angle, Normal, Short Telephoto, Medium Telephoto, Long Telephoto, and Super-long Telephoto. The exact ranges of these groups are debatable.

Some common misconception.

Telephoto lens = physical length of the lens is shorter than the focal length.

ie, Canon 100mm f2.8 lens is NOT a telephoto lens(physical length = 109mm)

mansalim wrote in post #11897444
Telephoto lens = physical length of the lens is shorter than the focal length.

ie, Canon 100mm f2.8 lens is NOT a telephoto lens(physical length = 109mm)

While what you say is technically true, the extreme majority of photographers that I have interfaced with over the last 50 years or so have generally used the term "telephoto" to refer to any lens with a focal length longer than the "normal" focal length for a given camera format.

In common photography parlance, even though it isn't quite technically correct, "telephoto" is roughly the opposite of "wide-angle".

SkipD wrote in post #11897461
While what you say is technically true, the extreme majority of photographers that I have interfaced with over the last 50 years or so have generally used the term "telephoto" to refer to any lens with a focal length longer than the "normal" focal length for a given camera format.

In common photography parlance, even though it isn't quite technically correct, "telephoto" is roughly the opposite of "wide-angle".

i think the term is soldered to our minds based on the fact that majority of lens longer than 50mm incorporate telephoto design, to keep the size smaller.

SkipD wrote in post #11897461
While what you say is technically true, the extreme majority of photographers that I have interfaced with over the last 50 years or so have generally used the term "telephoto" to refer to any lens with a focal length longer than the "normal" focal length for a given camera format.

Sorry, but it's outright incorrect. mansalim's definition is correct. For what it's worth, the terms "long lens" and "medium lens" are available.

As for your numerous posts on "crop factor", I would draw your attention to the EXIF tag "35mm equivalent focal length". It's an accepted enough term for cameras to actually multiply by their crop factor and stick it in the image metadata. I have no problem with the term "equivalent focal length" (with the "35mm" being implicit). As an ex-medium format shooter, I'd rather the convention had settled on angle of view (2 required for each lens of course), but it is what it is and "35mm equivalent focal length" is established and useful.

Hehe, these just keep coming back... The whole idea of imagining another focal length to compensate a crop factor is becoming obsolete since less and less people are being brought up on 35mm film. In the past (more and more distant past every day), there were many film SLR kits with a 50mm prime as the main and only lens. If you want a lens for your 1.6 crop body that'll fit the exact same subject in the frame from the same shooting position, we're talking about a full frame equivalent focal length. It's used all over the place (even in compact cameras).

While it may not be technically correct - as some members insist on reminding us of - it's pretty much the easiest way to know in advance what sort of image is gonna hit your sensor if you compare the focal lengths to a film body (or FF for that matter).

Other than that... it's just... cropped. Try opening an image and cropping about half out of it. By far the best insight you can get into the difference between FF and crop. Nothing changes in focal lengths, but the edges of the image are "clipped" since there's no sensor around those parts. Consider it like moving a slideshow projector forward.