Go Back   Steve's Digicams Forums > Digicam Help > What Camera Should I Buy?

Reply
 
Thread Tools Search this Thread
Old Jul 14, 2010, 9:27 AM   #1
Junior Member
 
Join Date: Jul 2010
Posts: 5
Default canon xsi vs t1i

Ok, so I was pretty sure on my choice to find a used xsi body on ebay or something for around $350 and buy a few lenses (canon 50mm f/1.8 II, and canon 28-135mm f/3.5-5.6 IS USM or the Tamron AF 28-75mm f/2.8 XR Di LD...still debating)...Just a low cost entry SLR to get started with. At least until I came across something TCav posted on another topic here:

"Ok, but was there a little girl flying across the room in the picture you took? You want to take photos of your daughter performing gymnastics. For that, you need faster shutter speeds. When you use faster shutter speeds, you don't need to wory about motion blur. In order to use fast shutter speeds, you need larger apertures and higher ISOs. A camera with a maximum ISO setting of 1600 (like the XSi) means you can only use lenses with a maximum aperture of f/2.0 or larger (numerically smaller). A camera with a maximum ISO setting of 3200 (like the T1i) means you can use lenses with a maximum aperture of f/2.8.

You can go with the XSi, but your lens choice is limited to fast primes. If you go with the T1i, you could use a zoom like the Sigma 50-150/2.8 or the 70-200/2.8 from Sigma, Tamron or Canon"

Being new to the whole SLR thing, Ill have to say that Im slightly confused. If I go with the xsi would the lenses I am considering being limited in their operating specifications (mainly the Tamron since it's f/2.8)? Would someone mind elaborating on how ISO settings and aperature sizes relate?

Thanks in advance!

James
megfourfun is offline   Reply With Quote
Sponsored Links
Old Jul 14, 2010, 9:47 AM   #2
Moderator
 
Join Date: Aug 2004
Posts: 8,529
Default

Quote:
Originally Posted by megfourfun View Post

Being new to the whole SLR thing, Ill have to say that Im slightly confused. If I go with the xsi would the lenses I am considering being limited in their operating specifications (mainly the Tamron since it's f/2.8)? Would someone mind elaborating on how ISO settings and aperature sizes relate?

Thanks in advance!

James
James - the quote you used was discussing gymnastics. The key there being the need for high shutter speeds. If a subject is moving and you have a slow shutter speed the subject shows motion blur. The faster the shutter speed the more you can freeze motion.

Shutter speed, aperture and ISO all work together to determine exposure. I'll elaborate in another post.

What you have to learn right up front is NOTHING you buy short of spending $20,000 will be capable of shooting every possible thing you might think of.
JohnG is offline   Reply With Quote
Old Jul 14, 2010, 9:51 AM   #3
Moderator
 
Join Date: Aug 2004
Posts: 8,529
Default

Here is a write-up I did a while back directed at someone wanting to shoot sports. The majority applies to everything though - there are just references to 'fast shutter speeds' which are necessary for sports but not other things. While 1/400 may be needed for sports, it's certainly not needed for a photo of a person sitting at a table.
Exposure is controlled by 3 variables:
1. Aperture – how much light gets to the image sensor
2. Shutter speed – how long the sensor is exposed to the light
3. ISO – how sensitive the sensor is to that light

Aperture – how wide the iris in the lens opens to let light in. Measured in f-stops. The f-stop is a ratio of the opening to the physical focal length of the lens. Because it’s a ratio, if the focal length stays the same, and the opening gets wider, the ratio (or f-stop) gets smaller.

What you need to know off the bat is – the smaller the f-stop the wider the opening and thus the more light that gets in. There is another resulting affect – depth of field. The wider the aperture for a given focal length, the less of the image will be in focus. The narrower the aperture for a given focal length (i.e. the larger the f-stop) the more of the image will be in focus.

Read here to get an understanding of depth of field:
http://www.luminous-landscape.com/tu...ries/dof.shtml

F-stops are measured in FULL STOP or PARTIAL STOPS (usually 1/3 stop).
Full stop values: F1.0, f1.4, f2.0, f2.8, f4.0, f5.6, f8.0, f11, f16, f22, f32
(there’s a pattern that makes it easy to remember – every other stop is a doubling f1 – f2 – f4 – f8 – f16 vs. f1.4 – f2.8 – f5.6 – f11 – f22)

Shutter speed – how long the shutter stays open and light hits the sensor. It is measured in seconds. In most cases the shutter is open far less than a second – so the notation is typically 1/x. Shutter speed is also often referred to in STOPS. Full stop values are typically 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000, 1/8000. Notice every STOP is a doubling. The camera is usually capable of changing shutter speeds in 1/3 stop increments though.
Shutter speed is important for sports shooting because it is shutter speed that determines whether or not you see motion blur. Different types of motion require different shutter speeds to completely freeze. A major league pitcher throwing 95mph requires a very different shutter speed to freeze than a jump shot in basketball.

In most cases, as a sports shooter, faster is better. There are exceptions, but in general you want faster shutter speeds. For many human sports, 1/500 is considered the minimal shutter speed you want to have. For softball, however 1/500 will show a lot of blur in pitching, hitting and throwing.

ISO – how sensitive the camera’s sensor is to light. Also measured in full and 1/3 stops. Full stop values are 100, 200, 400, 800, 1600, 3200. Notice again every stop is a doubling of value. ISO 200 is twice as sensitive to light as ISO 100. The negative affect of higher ISO though is you get a lot of digital noise in the images.

Now for the fun part – how they all work together.

Think of proper exposure as the image looking good – not too dark, not too light. Let’s say you take a photo that is properly exposed.
Let’s say the 3 variables for that photo are: ISO 200, f8.0 and 1/250.
Now, those exact settings aren’t essential for the proper exposure – what is essential is their RELATIVE value to each other.

You can change the sensitivity of the sensor (ISO) up or down. If you change it up without changing the other two values, the image will be too bright. If you change it down without changing the other two values the image will be too dark.
The same holds true if you change only aperture (fstop) or change only shutter speed.

BUT, if you change ISO UP and one or both of the other two variables DOWN you’ll get the same exposure.
SO, original shot was ISO 200, f8.0 and 1/250
If you move ISO up 1 stop to 400, to get the exact same exposure you need to move one or both of the other 2 variables DOWN by a total of 1 stop.

So ISO 200, f8.0 and 1/250 has same exposure as ISO 400 f16 1/250 has the same exposure as ISO 400 f8.0 1/500

Similarly if I bump ISO up from 200 to 1600 (3 stops), I can change the other 2 values by a total of 3 stops. So I could gain 3 stops of shutter speed –
ISO 200 f8.0 1/250 has same exposure as ISO 1600 f8.0 1/2000


So, how do you decide which combination of these 3 variables you want?

Ah hah,

In general, for this type of sports shooting you want wide apertures (so you get shallow depth of field – so you see your subject in focus but the background blurred) and you want high shutter speeds so you freeze action.

To that end, for outdoor sports, photographers often shoot with f2.8 lenses set to f2.8.
To gain faster shutter speeds they will increase ISO until they get a fast enough shutter speed to freeze the type of motion in question. Experience will tell you what that shutter value is.
JohnG is offline   Reply With Quote
Old Jul 14, 2010, 10:15 AM   #4
Junior Member
 
Join Date: Jul 2010
Posts: 5
Default

Thanks JohnG. Clear and concise overview of the 3 exposure variables. TCav's post makes more sense now and I think I can go back to my original plan.
megfourfun is offline   Reply With Quote
Old Jul 14, 2010, 9:27 PM   #5
Senior Member
 
Join Date: Oct 2009
Posts: 350
Default

Quote:
Originally Posted by megfourfun View Post
Thanks JohnG. Clear and concise overview of the 3 exposure variables. TCav's post makes more sense now and I think I can go back to my original plan.
You are saying you won't want to take gymnastics photos (with the xsi)?
javacleve is offline   Reply With Quote
Old Jul 14, 2010, 9:35 PM   #6
Senior Member
 
shoturtle's Avatar
 
Join Date: Dec 2009
Location: Frankfurt AM
Posts: 11,348
Default

The xsi does not have high enough iso to shoot in a low light gym even with very bright lenses with action. You will need need 1.8 or better with atleast 3200iso.
__________________
Super Frequent Flyer, no joke. Ex Patriot and loving it.
Canon Eos 60D, T1i/500D, Eos1, Eos 630, Olympus EPL-1, and a part time Pentax K-X shooter.
shoturtle is offline   Reply With Quote
Old Jul 15, 2010, 8:06 AM   #7
Junior Member
 
Join Date: Jul 2010
Posts: 5
Default

Quote:
Originally Posted by javacleve View Post
You are saying you won't want to take gymnastics photos (with the xsi)?
exactly. lol
megfourfun is offline   Reply With Quote
Old Jul 15, 2010, 8:20 AM   #8
Senior Member
 
TCav's Avatar
 
Join Date: Sep 2005
Location: Washington, DC, Metro Area, Maryland
Posts: 13,543
Default

Quote:
Originally Posted by megfourfun View Post
Quote:
Originally Posted by javacleve View Post
You are saying you won't want to take gymnastics photos (with the xsi)?
exactly. lol
So, yes, then the comments I made that you quoted from wouldn't apply to your situation.
__________________
  • The lens is the thing.
  • 'Full Frame' is the new 'Medium Format'.
  • "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.
TCav is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 7:23 PM.