Here's a breakdown to help you decide:
Factors to Consider:
* Subject Distance: This is the most crucial factor.
* 600mm: Excel at capturing distant subjects. Ideal for wildlife that's difficult to approach (e.g., shy birds, large mammals in a reserve), sports where you're far from the action (e.g., football, motor racing), and situations where you want significant subject isolation with a shallow depth of field.
* 400mm: Better for subjects that are closer or more approachable. Good for smaller birds, sports like baseball (where you might be closer to the field), portraits with a telephoto effect, and some wildlife encounters where you can get reasonably close.
* Crop Factor (Sensor Size):
* Full Frame: The focal length is what it is (400mm or 600mm).
* APS-C: A 400mm lens provides a field of view equivalent to approximately 600mm on a full-frame camera (1.5x or 1.6x crop). A 600mm lens becomes equivalent to a 900mm lens. This can be beneficial for reach but also exaggerates camera shake and finding subjects within the frame.
* Micro Four Thirds: A 400mm becomes 800mm equivalent, and 600mm becomes 1200mm.
* Weight and Size:
* 600mm: Generally larger and heavier. More challenging to handhold for extended periods and require a robust tripod and gimbal head for optimal stability and image sharpness. More difficult to travel with.
* 400mm: More compact and lighter, making them easier to carry and handhold (although stabilization is still recommended). Better suited for travel and situations where mobility is important.
* Price:
* 600mm: Generally significantly more expensive. High-quality 600mm lenses are a substantial investment.
* 400mm: More affordable, especially when comparing comparable quality levels.
* Aperture:
* This varies depending on the specific lens. Generally, both 400mm and 600mm lenses come in a range of aperture options. Wider apertures (e.g., f/2.8 or f/4) allow for more light, faster shutter speeds, and shallower depth of field (more subject isolation). However, wider apertures often come at a higher price and increased weight.
* Image Stabilization (IS/VR):
* Essential for both focal lengths, especially when handholding. Look for lenses with effective image stabilization to minimize camera shake. Newer lenses often boast 5+ stops of stabilization.
* Autofocus Performance:
* Critical for capturing fast-moving subjects. Invest in a lens with fast and accurate autofocus. Consider the autofocus system of your camera body as well, as it will significantly impact performance.
* Shooting Environment:
* Open Spaces: 600mm can be beneficial in wide-open areas where subjects are distant.
* Confined Spaces: 400mm might be preferable in denser environments like forests or zoos, where you might not have enough room to frame your subject with a 600mm lens.
* Versatility:
* 400mm: Offers more versatility. It's more manageable for a wider range of subjects and situations. Can often be paired with a teleconverter to extend reach when needed.
* 600mm: More specialized. Best suited for dedicated telephoto work.
Pros and Cons Summary:
400mm Lens:
* Pros:
* More affordable
* Lighter and more portable
* Easier to handhold
* More versatile for different subjects
* Can be combined with teleconverters for increased reach
* Cons:
* May not provide enough reach for very distant subjects
600mm Lens:
* Pros:
* Excellent reach for distant subjects
* Stronger subject isolation
* Ideal for professional-level wildlife and sports photography
* Cons:
* Significantly more expensive
* Heavy and bulky
* Requires a sturdy tripod and gimbal head
* Less versatile for different subjects
When to Choose a 400mm:
* You're on a budget.
* You prioritize portability and ease of handling.
* You photograph a variety of subjects at varying distances.
* You primarily photograph subjects that you can approach reasonably closely.
* You want a more versatile lens for travel.
* You're willing to use a teleconverter to extend reach when needed.
When to Choose a 600mm:
* You specialize in wildlife, sports, or bird photography where subjects are typically distant.
* You need the maximum reach possible.
* You're willing to invest in a high-quality, specialized lens.
* You have a robust tripod and gimbal head to support the lens.
* You don't mind the increased size and weight.
* You rarely photograph subjects at closer distances.
Recommendation:
* Beginner/Hobbyist: Start with a 400mm lens. It offers a good balance of reach, price, and portability. You can always add a teleconverter later if you need more reach.
* Serious Enthusiast/Professional: If your primary focus is capturing distant wildlife or sports subjects, and you have the budget and equipment to support it, a 600mm lens is a worthwhile investment.
Before You Buy:
* Rent Before You Buy: The best way to determine which lens is right for you is to rent both a 400mm and a 600mm and test them out in your preferred shooting environments.
* Consider Your Camera Body: The performance of a lens is also heavily influenced by the camera body it's paired with. Make sure your camera body has a capable autofocus system and can handle the demands of a long telephoto lens.
* Read Reviews: Research reviews from reputable sources before making a purchase.
Ultimately, the choice between a 400mm and a 600mm lens depends on your individual needs, budget, and shooting style. Carefully consider all the factors outlined above before making your decision. Good luck!