Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
11-14-2011, 02:07 AM   #1
Senior Member




Join Date: Feb 2011
Location: connecticut
Photos: Gallery
Posts: 248
f2.8 - is it different for a macro lens vs a sports long zoom?

if f2.8 has such a narrow depth of field in most macro and close up shots, why is it desirable in a long zoom , say the 70-200 range? does the depth of field change to feet instead of inches ? sorry if this is a dumb question.

11-14-2011, 02:18 AM   #2
Veteran Member
TOUGEFC's Avatar

Join Date: Nov 2009
Location: Brisbane
Photos: Gallery
Posts: 3,561
Its simple really, the further away your subject, the greater the depth of field is
11-14-2011, 04:52 AM   #3
Loyal Site Supporter




Join Date: Mar 2009
Location: Gladys, Virginia
Photos: Gallery
Posts: 15,755
Depth of field is dependent on several things: aperture, focal length, sensor size, and distance to subject. Take a look at this link: Understanding Depth of Field in Photography.
11-14-2011, 05:24 AM   #4
Forum Member




Join Date: Apr 2008
Location: Adelaide
Photos: Albums
Posts: 98
The main reason that a large maximum aperture is desirable in a long zoom is that it lets in more light and allows a faster shutter speed. That way you are less likely to get blur from camera shake.

The effect of any camera shake is magnified the longer your focal length. If you move an 800mm lens 2 cm it could mean your subject is no longer in view if it's far enough away. That's why the old rule of thumb exists: set your minimum shutter speed to equal your focal length in mm or focal length /shutter speed eg 50mm and 1/50th or 200mm and 1/200th.

It's also worth remembering that lenses are always sharper after stopping down 2-3 stops. That means (assuming everything else is equal) you will get razor sharp images at f5.6 from a lens with a maximum aperture of 2.8 and not so sharp images from an f4.0 lens.

11-14-2011, 08:46 AM   #5
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,396
As far as I am concerned, there are 2 differing reasons for large apertures when considering the difference between a telephoto for sports, and a macro.

As others have stated, since depth of field is a function of focusing distance, although a 200mm F2.8 lens, for example has a relitively narrow depth of field, it is considerably more when focused at perhaps 50-100 meters than a macro lens focused to 50-100mm

For a macro, aside from possible (but in my view negligable ) artistic differences between an F2.8 and F4 macro when wide open, the big issue is available light for focusing when you are dealing with high enlargement ratios. At 1:1 macro, the viefinder can get very, very dark, and an additional stop of light is a big benefit. Many people argue that F4 is all you need for a macro because most times you are going to stop down when exposing for DOF, but they may have never tried to focus one at 1:1

For sports, it is all about shutter speed and motion freezing capability. F2.8 lenses are 1-2 stops faster than consumer tele lenses, which can give you 2-4 times the shutter speed without increasing ISO. 2-4 times shutter speed is the advantage here,

Different needs different requirements in a lens
11-14-2011, 09:12 AM   #6
Pentaxian
JinDesu's Avatar

Join Date: Jun 2011
Location: New York City
Photos: Gallery
Posts: 5,626
F2.8 is desirable in a long zoom for another reason - if I am indoors and far away, I wanted the largest aperture possible in order to increase my shutter speed so I have no camera shake in the pictures.
11-14-2011, 10:42 AM   #7
Veteran Member
RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
A fast AF macro lens is great for non-macro shoots, although it may be sharper at closer rather than further distances. You don't need a wide aperture and AF for macro shoots -- you need good manual control of everything. My old M42 Vivitar-Komine 90/2.8 macro only gets used wide-open for portraiture and other short-tele work. At macro distances, it needs to be stopped-down. Some of the better macro lenses from various makers, those intended for macro and not general use, may be slower: f/3.5-4.0.

As mentioned, there are other reasons a fast AF tele is desirable -- DOF control and action and more light. Long fast AF teles for sports and wildlife are not small nor cheap. Yet small differences can make a vast difference in price. My old M42 Sears-Tokina 55-135/3.5 and Vivitar-Kiron Series 1 70-210/3.5 are smaller and much cheaper than their f/2.8 counterparts, yet are only slightly slower. What you need in a lens depends on what you're doing with it, eh?

Last edited by RioRico; 11-14-2011 at 10:53 AM.
11-14-2011, 11:20 PM   #8
Senior Member




Join Date: Feb 2011
Location: connecticut
Photos: Gallery
Posts: 248
Original Poster
so, if i understand, a macro such as the 100 mm's have a better image quality for close up or macro, and are designed for that, than at longer distance , farther away subjects ? and macro lenses in general are to be manually focused for macro shots? and depth of field is such at 2.8 that most people will use f 4 or something around that, for macro?

11-15-2011, 03:05 AM - 1 Like   #9
Veteran Member
RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
QuoteOriginally posted by dh4412 Quote
so, if i understand, a macro such as the 100 mm's have a better image quality for close up or macro, and are designed for that, than at longer distance , farther away subjects ? and macro lenses in general are to be manually focused for macro shots? and depth of field is such at 2.8 that most people will use f 4 or something around that, for macro?
Well, yes, but... Some macro lenses do well at distance, but many are best at close range. Yes definitely, at macro ranges we need to focus manually -- AF just isn't precise enough. And yes but, while we need to stop-down (and maybe do some focus-stacking) to get macro shots with any decent DOF, some pervs DO shoot wide-open macros, just to exploit the very thin DOF artistically.

What makes a macro lens? 1) Design to focus very close, and 2) edge-to-edge flatfield sharpness. Enlarger lenses (ELs) have these qualities too, which is why I like using ELs on bellows and/or tubes. Some macro and enlarger lenses just don't do well at distance. Some, like my Macro-Takumar 50/4, are built so that focusing beyond 2m is difficult. The lens reviews will tell you which commercial macro lenses are weak afar.

Edge-to-edge flatfield sharpness is important if we're shooting flat stuff, maybe a bit less critical for centered 3D subjects. But if we want maximum sharpness throughout the frame, we need to 1) focus manually and carefully, and 2) stop-down for thick DOF. "Focus carefully" may include using focus rails: we slowly move the subject to the precise necessary spot. And for much stopped-down work, we need flash.

And flash is why we may need AF macro lenses. Pentax P-TTL flash only works with A-type and AF lenses. A-type manual-focus macro lenses aren't real common; AF macro lenses abound. Yes, non-P-TTL flash *can* be used with lenses lacking auto-aperture contacts, but it's trickier, with lots of chimping and adjusting. Some here are masters at that. And they put much time and effort into their brilliant results.

Hope this helps!
11-15-2011, 10:55 PM   #10
rfg
Senior Member




Join Date: Dec 2010
Location: Sydney
Posts: 137
QuoteOriginally posted by RioRico Quote
.... definitely, at macro ranges we need to focus manually -- AF just isn't precise enough.....
I can quite see this for phase/viewfinder AF, but does it also follow for live view/contrast AF? In general that seems more accurate but slower so I was wondering whether it might be good enough for macro work....?
11-16-2011, 05:18 AM   #11
Forum Member




Join Date: Apr 2008
Location: Adelaide
Photos: Albums
Posts: 98
It doesn't matter whether you are using live view or the viewfinder, AF is hit and miss for precise focusing. Macro just accentuates any errors.

The AF targets are not really an accurate enough indicator of where the AF sensor is focusing. You may have the sensor over an insects eye and think it's fine but the sensor may actually be focusing on it's jaw. With manual focusing you can adjust till what you want to be in focus is sharpest. It also makes using the hyperfocal distance easier.

It can be worth using manual focus for portraits for this reason too.
11-16-2011, 06:34 AM   #12
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,396
QuoteOriginally posted by rfg Quote
I can quite see this for phase/viewfinder AF, but does it also follow for live view/contrast AF? In general that seems more accurate but slower so I was wondering whether it might be good enough for macro work....?
It would only work if you add my favourite (and missing) function Mindread.exe

the issue with any AF system but definitely made much worse with macro is that it is impossible to tell the camera exactly what you want in focus.
11-16-2011, 07:15 AM   #13
Veteran Member
RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
Dark_Mist and Lowell are quite right. AF is probabilistic, not certain -- it *might* get the focus right. And it's areal, not pontillistic -- it *might* focus on the spot you desire.

These problems | characteristics are easy and cheap to test. Get a +10 dioptre closeup adapter for US$10-$15 and screw it onto a longish AF zoom, say a 50-200 or 55-300 or 70-210. That +10dpt adapter on a lens at 100mm gives 1:1 magnification; at 200mm, it's 2:1. Now autofocus on something and SNAP the shutter. See how well the AF'bot achieved the focus you intended. Is it anywhere close? Congratulations! You should be a lottery winner! Most of us will get results that are all over the place. Damn, another fuzzy ant...

That's also why I use MF lenses for portraiture. An 85/2 at headshot distance (say 1.5m) has DOF of 2mm. A USA penny is ~1mm thick. That means an eyeball may be in focus while the lashes (and everything else) are OOF. Even at 85/4, the DOF is only 5mm. That's thinner than a chopstick. Using AF for portraits gives lots of sharp noses and fuzzy eyes. If a specific spot *must* be focused dead-on, use MF.

Don't believe the numbers? Plug them in here: http://www.dofmaster.com/dofjs.html
11-16-2011, 12:31 PM   #14
Senior Member




Join Date: Feb 2011
Location: connecticut
Photos: Gallery
Posts: 248
Original Poster
i have noticed maybe 50% of my autofocus shots with any of my lenses ( about seven lenses ) have poor focus . sometimes more than that, 75% not focused; even with my manual lenses it's difficult to get really sharp focused shots more than half the time. so, why mess with autofocus at all?
11-19-2011, 11:48 AM   #15
Pentaxian
reeftool's Avatar

Join Date: Dec 2007
Location: Upstate New York
Photos: Gallery | Albums
Posts: 8,109
QuoteOriginally posted by Lowell Goudge Quote
As far as I am concerned, there are 2 differing reasons for large apertures when considering the difference between a telephoto for sports, and a macro.

As others have stated, since depth of field is a function of focusing distance, although a 200mm F2.8 lens, for example has a relitively narrow depth of field, it is considerably more when focused at perhaps 50-100 meters than a macro lens focused to 50-100mm

For a macro, aside from possible (but in my view negligable ) artistic differences between an F2.8 and F4 macro when wide open, the big issue is available light for focusing when you are dealing with high enlargement ratios. At 1:1 macro, the viefinder can get very, very dark, and an additional stop of light is a big benefit. Many people argue that F4 is all you need for a macro because most times you are going to stop down when exposing for DOF, but they may have never tried to focus one at 1:1

For sports, it is all about shutter speed and motion freezing capability. F2.8 lenses are 1-2 stops faster than consumer tele lenses, which can give you 2-4 times the shutter speed without increasing ISO. 2-4 times shutter speed is the advantage here,

Different needs different requirements in a lens
A good explanation Lowell, as usual. A bright viewfinder is always better, regardless of the subject matter. I would add that an f/2.8 (or faster) lens is usually a little better made and in many cases (but not all) will have a little better IQ stopped down.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
camera, depth, f2.8, field, macro, pentax help, photography
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
best mid to long range zoom lens sany Pentax SLR Lens Discussion 28 06-08-2011 07:59 AM
What's the best zoom for sports? Adam Pentax SLR Lens Discussion 29 02-21-2011 12:41 AM
Long Zoom lens question HoBykoYan Pentax SLR Lens Discussion 10 06-30-2010 06:42 PM
Shooting winter sports all day long. jbrowning Photographic Technique 10 12-18-2009 02:27 PM
Fast fixed or zoom lens for sports? Davidparis Pentax SLR Lens Discussion 14 11-13-2009 08:29 AM



All times are GMT -7. The time now is 12:24 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top