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I currently have a Canon 100mm 2.8L mounted on a cropped sensor camera, which makes it 160mm equivalent. I have two questions.

  1. Since the equivalent focal length is 160mm, does that mean that the minimum focusing distance increases to about 32cm?
  2. Is there an external device/tool I can use to magnify what I see through the viewfinder even further?
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1  
Focal length equivalent does not mean a focal length change exists. The lens is still a 100mm focal length, and its distance to the sensor is still the same. Magnification at the sensor is still the same, too. Beyond that, the "effective magnification" is how you handle it. A larger monitor "magnifies" your image, too. –  Skaperen Feb 4 '13 at 14:17
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It's better to ask two separate questions in two separate questions, because the answers might be different. –  mattdm Feb 4 '13 at 16:27
    
Maybe this should go without saying, but it's even better to ask three separate questions in separate questions. Please! –  mattdm Feb 5 '13 at 20:36

5 Answers 5

up vote 9 down vote accepted

The minimum focus distance is a property of the lens and the distance to the sensor. The lens-to-sensor distance is the same for all Canon DSLRs hence the minimum focus distance doesn't change when using a cropped sensor body.

There are adapters you can get to magnify the viewfinder image, here's a review of an offical Canon product, numerous third party alternatives are available. If you have a DSLR with liveview then you can magnify the image very easily using the camera settings.

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1. Your lens' minimum focusing distance does not change

2. By taking more from viewfinder if you mean more on to the sensor too? if so You can use extenders with the multipliers of 1.4 and 2 which canon is selling worldwide. Though if you want to photo further my advice to you is to get tele lenses. But if not the first answer of the friend wil guide you for viewfinder zooming.

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What would be the difference between using an extender as opposed to extension tubes, if any, on a macro lens? –  Bob Feb 4 '13 at 19:53
    
Extention tubes have no lens(glass element) inside which allows you to make close-up alike focusing capabilities. But by that you are losing you ability to focus to infinity. On the other hand extenders gives you a multiplier and preserves your lenses minimum focus distance and infinity focus capability. The extender's disadvantage is as I'm using canon's, 1.4 extender makes you lose 1 f stop and 2 extender makes you lose 2 f stops. –  Yiğit Hür Ulaş Feb 4 '13 at 20:14
    
I just double checked, and they are only compatible with the 180mm Macro, not the 100mm... –  Bob Feb 5 '13 at 0:35
    
Since it's night here I cant confirm 'cause I used to own a 100mm before I got my 2x extender, gotta answer that tomorrow to call a friend –  Yiğit Hür Ulaş Feb 5 '13 at 0:49

As everyone already said it: The minimum focus distance does not change.

Now, since the focus distance stays the same and the field-of-view is cropped by the FLM, you may be wondering about magnification too: Magnification does not change either.

This happens because magnification is measured relative to sensor-size. So you an picture something with a smaller angle-of-view at the set distance but it is reproduced on a smaller sensor. These two things cancel either out and you have the same 1:1 magnification except that you actually took a photo of something smaller!

On some cameras with optical viewfinders, you can can magnify what you see through the viewfinder with an eye-piece magnifier. Those let you see more clearly what is through the lens but wont tell you capture something smaller.

To capture something smaller you either need a diopter, sometimes called close-up adapter, which is basically magnifying glass that screws to the front of your lenses or an extension tube which moves the lens further from the sensor. A good number of such options have been discussed already here, do a search for macro or extension tubes.

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1) No. The minimum focusing point is a property of the lens. The only thing that changes on a "crop sensor" is the field of view.

2) What body are you using? The only thing I'm aware of is Electric Viewfinders a with digital magnifying feature.

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Finder magnifiers (essentially miniature telescopes) are available for just about all SLRs in both straight and 90-degree angle versions and various magnifications (including zooms). They usually restrict what you can see, showing only the central portion of the VF image, but they also usually hinge up out of the way for framing. (They were originally meant to be critical focus aids for manual focus cameras.) –  user2719 Feb 4 '13 at 8:47

First, the size of sensor DOES NOT affect any type of focusing aspect of any lens you attach. Lenses are the device that focus’ light. Sensors (or film) do not perform any focus operations. This concept applies to subjects including depth of field, minimum focusing distance for macro work, infinity focus and others.

All lenses are designed to do one thing; take incoming light and focus it to a point ON A FLAT PLANE inside the camera body. Lenses made by all manufacturers are designed to focus their light at a very specific distance. This distance, which goes by a couple different “official” names, is measured from the flange of the lens mount to the sensor.

The only thing that is affect by the amount of surface area you are trying to focus light onto (i.e. size of the sensor) is the angle of view of whatever lens you attach. If the limited angle of view experienced by a “crop” sensor camera forces you to change the distance from camera to subject, it is this change in distance that affects the aforementioned aspects of focusing. Not the crop sensor itself.

Your second question could be a little clearer. If you are talking about magnifying the image in a viewfinder, as previously mentioned you can attach a magnifying eyepiece adapter. Or, you could also go with a magnifier that attaches to the LCD screen on the camera back.

If you are talking about seeing more of the overall image than what your particular viewfinder shows – in other words, your model of camera only shows less than 100% viewfinder coverage – then, no adapter on the viewfinder itself will fix this. Depending on your model of camera, the LCD itself (in live view mode) may provide more coverage.

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