Does anyone know much about DSLR lenses?

Question

Basically, I’m looking to get a new camera, and will probably get an SLR. It’s the first time I’ve bought an SLR, so don’t really understand a lot of the terminology!

With my compact, I get a x4 zoom on it. With some of the high end SLR look-alike cameras I’ve seen, I can get x18 zoom – I understand all of that (sort of). But, when I look at a ‘proper’ SLR, it doesn’t seem to use the xZoom rating, rather mm, and was just wondering how these translate into xZoom equivalent! For example, 1 camera I looked at comes with an 18-55mm lens & a 55-200mm lens and wondered how that compared with a x18 zoom camera.

thanks

Answer 1

It depends on the camera you are looking at.

For a digital slr that has a sensor the same size as 35mm film, the lens sizes are the same as for a 35mm camera. Which is around 1x=50mm. Take into consideration that an 18x zoom is going to be a digital zoom, not optical. Optical zoom is much higher quality. Digital zoom is nothing more than the camera using software to blow up the picture. Also, the digital sensor in a dslr is larger than in a compact camera, and also than the one in the slr look-a-likes. It also uses a different sensor technology which makes for better pictures.

Answer 2

Since SLR cameras have interchangeable lenses - they are rarely judged by their zoom. Rather by the focal length of a particular lens.

The zoom lenses are usually marked with shortest and longest focal length. So 18-55mm lens goes from 18mm to 55mm.

18mm is a standard wide angle that should be able to capture a group of people within a room. 55mm is a good zoom level for a portrait shot, again from across the room. 200mm is a pretty long telephoto. Good enough to capture a single person from across the street....

Since you can have more than one lens, the zoom ratio is not as important, but if you want to translate it into P&S terms, simply divide the larger number by smaller. So 18-55 lens is 55/18=3x, 55-200 is approximately 200/55=3.6x

It is usually considered that lenses up to 4x are the best, because when the large range is used it is harder to correct all kinds of distortions. However in todays world there are some good long zooms. For example Nikon has a very nice (albeit expensive) 18-200 lens (that's 11x). While the lens does have some distortions, they are not huge and are correctable in photoshop if need be.

Last but not least - if you are considering a long lens, longer than 100mm (or even less) consider Vibration Reduction (also known as Image Stabilization). This is a feature that compensates for the natural hand instability on long focal length shots. It is a lot harder to shoot handheld without it. Though it does cost extra money.

LEM.

P.S. Keep in mind that third party lenses like Sigma or Tamron are significantly poorer in quality than ones made by original camera manufacturers such as Nikon or Canon. So try not to save money and buy the original brands! Remember lenses are a long-term investment. While your body will probably become absolete after a few years, the lenses can serve you literaly for decades, as they will probably work just as well with newer model bodies you will buy in later years.

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Hi, anthony!

You have a point in the fact that there are some unique lenses made by 3rd party manufacturers, but please allow me to re-state my ground that Sigma and Tamron produce clearly inferrioir lenses to Nikon and Canon. Tokina is a bit better (I never mentioned it above, but they do come at a cost)

Here's what Ken Rockwell has to say about Sigma 10-20 you've been praising (compared to similar, though not identical but general super wide angles:

http://kenrockwell.com/tech/digital-wide...

As you can note, Ken singles Sigma lens as lowest quality, while Nikkor as highest.

I am yet to find a single professional review that concluded that Sigma glass is optically of same or superior quality to Nikon or Canon glass. Usually the reviews say exactly the opposite. Not even close! If you don't mind, please site a source of your information! Otherwise why would anyone pay twice the money for Nikkor optics???

Answer 3

The reason that SLR lens don't give you the Zoom ratio is because it is, essentially, meaningless.  For any specific camera model any lens with a specific mm length will give the same image magnification.  The Zoom Ratio is just the value of the shortest (lowest number) focal length  divided into the longest (highest number) of the lens.  For example -

A 10-100mm, 20-200mm and 50-500mm are all 10x zooms, but if you get a 10-100mm for a typical DSLR it will be a "wide angle" to moderate telephoto while a 50-500mm lens used on the same camera will be a "normal" or slight telephoto to a "super telephoto".

The other factor for Digital SLRs is the size of the image sensor.  Compact cameras generally use much smaller image sensors than DSLRs.  The smaller the sensor the lower the mm measurement is to define a "normal" lens so the greater the magnification achieved at any given mm setting, but the image quality will be compromised if the sensor is too small.  

35mm film cameras use a standard size film format (24x36mm) and for many years this was the basis for defining advanced amateur and professional grade "standard format" cameras. For "standard" DSLRs (there is no standard because every company can choose their own sensor design)  the image sensors are sized to be anywhere from about 1/2  the size of 35mm film cameras to the 24x36mm "full frame" size.  This means that a "crop factor" is often used when comparing 35mm to digital lens sizes.

General guidelines for lens focal lengths (mm) and their definitions are -

Divide by the "crop factor" to get the equivalent sizes for a given DSLR camera.  

35mm Film/Full Frame                      

   6 -    21 mm   "Fish eye" to Super wide angle

24 -    35 mm   Wide Angle

38 -    60 mm   Normal (About what humans normally see)

75 -  200 mm   Moderate telephoto

250 - 500 mm   Long telephoto

Over  500 mm  Super-Telephoto

For additional guidelines for buying a DSLR you can go to this website which explains the terminology and helps first time DSLR buyers to figure out good choices.  

HTH, Good luck.  

Email if you have specific questions about a lens or camera choice.

Answer 4

All camera lenses will have a focal length measured in mm, even point and shoot lenses.  If you check the specs for a camera with a built-in lens, you'll find the lens equivalent in 35mm film cameras posted, because 35mm cameras were a standard for so long.  To find the specs in mm for any camera, visit the camera database at http://www.dpreview.com

Measuring the equivalent in 35mm terms is the most accurate way to do a comparison, because it equalizes everything, which the xZoom measurement doesn't do.  Here's why:

To get an xZoom measurement, you take the big # and divide it by the smaller one.  So 55mm/18mm = 3.05x, which means it's a 3x lens.  However, 200mm/55mm = 3.64x.  If you just compared the xzoom powers, the lenses would appear to be nearly identical (between 3x and 4x), yet they are very different lenses, with the first lens being wide angle to mild telephoto and the second lens being a telephoto zoom.  They have very different looks through the viewfinder.

So, like I said, convert everything to a 35mm equivalent.  For a digital SLR, multiply the lens by 1.5, so an 18-55mm has a 35mm equivalent view of 27-82.5mm

Now, for comparsion:  the Canon A570 IS, which has a 4x zoom.  Look at the lens specs here:

http://www.dpreview.com/reviews/canona57...

35mm to 140mm equivalent lens per the specs.

So, the 18-55mm spec again: 27-82.5mm   This means the 18-55mm lens will be wider (27mm is wider than 35mm), but have less telephoto power (82.5mm isn't as powerful as 140mm).  Adding in the 55-200mm, which is 82.5-300mm equivalent--the two lenses cover more telephoto now too from 27.5mm to 300mm (vs. 35mm to 140mm).

Now, let's look at an 18x zoom, the Panasonic FZ18:

http://www.dpreview.com/reviews/panasoni...

Lens specs:  28-504mm

The 18-55mm covers 27-82.5mm.

The 55-200mm covers 82.5-300mm

Bottom line:  the two lenses won't be as powerful, but it will still cover a very wide range (200mm / 18mm = 11.1x)

Finally, while LEM is mostly correct (and a thumbs up to him), he is somewhat mistaken about Sigma/Tamron/Tokina third party lenses.  The fact is that those companies make some unique and very good lenses.  For example, while Nikon makes an 80-400mm VR  lens, Sigma makes an 80-400 optical stabilized lens with a built-in motor.  Optically, the Sigma is the equal or superior of the Nikon.  Yet it's far less expensive!  Another example:  Only Tamron makes an 18-250mm lens (an equivalent 13.8x zoom) for SLRs.  Yet another example:  the Sigma 10-20mm is relatively inexpensive, yet performs fantastically for wide angle and I've seen this lens used by pros for interior photos because it doesn't distort.

So, I disagree with LEM on third party lenses.  If you know what you are doing, it's possible to get superb third party lenses, including lenses you can't find or get from the big camera companies, for a lot less money.